1. Introduction

Laser sparks can be advantageous relative to conventional sparks (i.e. those produced by widely used capacitive spark plugs) in ignition applications. First, there are differences related to the physical configurations, e.g. the fact that conventional spark discharges require the presence of adjacent electrodes which act as heat sinks and tend to quench the flame. Furthermore, the differing plasma parameters (initial temperature, pressure, electron parameters etc.) result in fundamental differences including flame propagation speeds. For example, a study of laser ignition of propane-air mixtures found the laser ignited flame speeds (at early times) to exceed the laminar flame speed, thereby providing a clear indication of plasma-assisted flame propagation [1]. Early flame speeds are particularly critical as this is when flame kernels are most prone to extinction due to flame stretch. Laser ignition (or ignition enhancement) is being considered in applications including reciprocating engines [2–4], ground based turbines, aero-turbines, rocket engines [5], and scramjet engines [6]. There is particular interest in the use of laser ignition for stationary gas engines, as are used for power-generation and gas compression, owing to the possibility of increased engine efficiency and reduced emissions. Figure 1 shows NO_x emissions for laser, spark plug, and prechamber ignition for a natural gas engine. For each means of ignition, as the air-fuel ratio increases, the NO_x reduces [7]. Laser ignition provides the lowest NO_x of all cases studied (note that the red and blue squares are calculations, while red and blue circles are measurements). There is also interest in using laser plasmas for ignition of turbines used in aircraft engines [8, 9] primarily in order to achieve rapid relight [8, 10, 11], to capitalize on the possibility of more optimal spark locations along the centerline of the combustor or in flow reversal zones near the fuel nozzle [12, 13], and to avoid the reliability limitations of conventional igniters.

 

Fig. 1 NOx emissions for laser (yellow circle), spark plug (green), and prechamber (PC) (red circles and blue circles) for a single-cylinder research engine (from [7]). The engine's coefficient of variation (COV) of peak pressure is held constant at 2% to ensure consistent test conditions.

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Despite its potential advantages, laser ignition is not currently used in commercial or industrial combustion systems. In some application areas, for example automotive, cost is a key factor. However, in applications such as large industrial engines and turbines for power-generation or aircraft, the cost of solid state pulsed lasers can be viable. In all practical applications, the overall system must meet requirements of performance, reliability, durability, safety, and cost. The majority of laboratory research has employed open-path beam delivery using mirrors to transmit the laser pulse to the combustion volume. Over short distances such configurations may be feasible but in many cases this type of beam delivery is impractical, for example for use on large industrial engines where there are many ignition locations, or in any application where the laser must be remotely located and transmitted over a relatively long path length or one where there is significant vibration or thermal drift of hardware. For such cases, three general system architectures are being considered. The first approach is based on reliable and compact laser systems that can be mounted in close proximity to the ignition location. Several diode pumped solid-state lasers (using both side- and end-pumping) with passive Q-switches have been developed for this purpose (e.g [14, 15].) including ceramic gain materials [16, 17]. The second approach is to use a single remotely located pump source (power ~300-600 W) that is transmitted through optical fiber to gain element(s) located at the ignition site(s) [7, 18, 19]. The third method, which is the focus of this contribution, is to have a single remotely located laser source and to deliver the high peak-power (~MW) pulses to the individual ignition location(s) via fiber optics. The approach is immediately attractive owing to its potential simplicity (low-cost) but the needed fiber delivery is technically challenging.

In applications where there are multiple ignition sites (e.g., 4-20 engine cylinders), a distributed approach as schematically shown in Fig. 2 with a single laser and fiber optic delivery may be preferred [4, 8, 20–22]. Such an approach could be advantageous because only a single laser source is needed and it could be positioned away from the increased temperature and vibration of the combustion location; however, fiber delivery has been challenging. Typical ignition laser sources have peak power of ~1-10 MW which is much higher than is typically used for fiber delivery and spark formation imposes the additional requirement of high beam quality (spatial-quality) at the fiber exit so that the light can be refocused to form a spark. Several researchers have concluded that fiber delivery is intractable or very challenging [19, 20, 23, 24] and indeed success with conventional step-index fibers has been limited.

 

Fig. 2 Schematic diagram of fiber delivered laser ignition from a single laser to multiple engine cylinders. The laser comprises a pump source and oscillator while a multiplexer is used to route the beam to different fiber channels (from [19]).

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The present contribution addresses the technical requirements and challenges of fiber delivery, summarizes past research, and highlights recent findings showing spark delivery and ignition that have occurred since publication of past reviews [19, 23, 24]. For multi-cylinder engines the fiber delivery should be combined with a multiplexer to distribute the individual source to multiple fibers. The multiplexing may be based on galvanometers [25], mechanically rotating optics [18], modulators [26], or compact scanners as are used for laser displays, but this aspect is beyond the scope of the current contribution. The layout of the remainder of the paper is as follows. The basic requirements for fiber output parameters are discussed in Section 2. Research efforts using hollow core fibers, step-index fibers, photonic crystal and bandgap fibers, and fiber lasers are presented in Sections 3-6 respectively. Finally, short conclusions and outlook to future work is presented in Section 7.

2. Fiber output parameters and basic considerations

To enable ignition, the fiber output must allow formation of a laser induced plasma with sufficient energy. While resonant schemes and thermal ignition are of long term interest, we fix the discussion by considering widely used non-resonant breakdown with nanosecond duration Nd:YAG lasers (1064 nm). The fiber must be able to reliably transmit high peak-power (megawatt) pulses with sufficient beam quality (low M²) to allow refocusing of the output beam to an intensity exceeding the breakdown threshold of the gas, i.e., I_BD,_Air≅100-300 GW/cm² for 10 ns, 1064 nm pulses at atmospheric pressure, and scales with pressure as ~p^-0.5 [27, 28]. For reciprocating engines, the motored pressure at time of ignition may be of order 10 bar with mixtures generally have relatively high air volume fractions, for example in the range of >~90% for lean burn natural gas engines. On the other hand, for aero-turbines the pressures can be in the vicinity of 0.2 bar so that higher focused intensities are needed. The aero-turbines also typically employ two-phase mixtures with breakdown intensities for the liquid droplets being only ~1 GW/cm² [29, 30]; however, typical droplet volume fractions are low enough that the focused beam generally does not overlap a droplet.

Comparing the needed focused intensity to the breakdown intensity of the fiber material allows one to recast the problem as a demagnification requirement. For widely used silica fibers the damage intensity is ~1-3 GW/cm² for 1064 nm radiation of ns duration [8, 23, 31, 32]. (Note that the fiber damage intensity is much lower than that for bulk silica which can be as high as 475 GW/cm² [33] for analogous conditions.) Given the ratio of the breakdown (damage) intensities, one finds that spark formation in (atmospheric pressure) air without damaging the fiber requires the output light be imaged to the spark location with linear demagnification of ≥10-20, i.e. the light in the desired spark region must be focused to a dimension much smaller than the fiber core. This requirement has been very difficult to meet with conventional multimode silica fibers. The fundamental problem, as has been recognized by several researchers [4, 8, 19, 21, 23], is that relatively large core sizes (>~100 μm) are needed to carry the required pulse energy (see below), but modal dispersion in the large core fibers then leads to degraded beam quality (elevated M²) at the fiber output which precludes tight re-focusing. From the Lagrange invariant, the demagnification is inversely proportional to the angular divergence of light exiting the fiber or, equivalently, the fiber exit beam quality (M²) [4, 34]. The need for high output beam quality immediately suggests the use of single mode fibers but, given their small core size (~3 µm – 30 µm), lens aberration makes it difficult to achieve sufficiently small focused spot sizes (on the order of 1 μm).

In addition to allowing spark formation, ignition requires that the plasma energy (absorbed from the laser) exceed the minimum ignition energy (MIE). MIE varies with applications but we generally consider the case of lean natural gas engines for which one has MIE of ~10-20 mJ [19, 23, 35]. Energy requirements for aero-turbines tend to be higher, for example some basic studies show required energies of ~30-60 mJ for reliable ignition [36], while other experiments in more realistic rigs use in excess of 100-200 mJ [11, 37–39]. In implementations that use a window to access (seal) the combustion volume, an additional constraint on the focusing configuration is the need to have an optical fluence that is sufficiently high to maintain window cleanliness (through laser self-cleaning) but sufficiently low to not damage the combustion window, which corresponds to a fluence in the range of ~0.5 – 10 J/cm² [40].

3. Hollow core fibers

Coated hollow core fibers have been demonstrated for spark delivery and laser ignition of a gas engine. As shown in Fig. 3, the fibers used in these experiments were cyclic olefin polymer-coated silver hollow fibers developed and manufactured at Tohoku University (Japan) [41, 42]. The hollow fibers were originally developed for delivery of mid-infrared lasers such as CO₂ (λ = 10.6 μm) and Er:YAG (λ = 2.94 μm), which cannot be delivered by silica glass fibers because of absorption loss. The coated fibers are flexible and have typical inner (hollow) diameters of 500-1000 μm and lengths of several meters. The maximum temperature the fibers can withstand is ~500 K which is reasonable for most targeted environments, though lifetime and reliability needs to be more fully considered [43]. Note that uncoated hollow fibers (capillaries) can also be used for light delivery, but they tend to have low transmission and to be extremely susceptible to bending loss [23].

 

Fig. 3 Left: Schematic diagram of coated hollow fiber (from [21]). Right: Photograph of spark formation at output of coated hollow fiber (from [21]).

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Spark formation in air at the output of coated hollow fibers has been demonstrated [4, 21, 44]. A single lens was used to launch laser light from a Q-switched Nd:YAG (1064 nm) into the fiber while a lens pair was used to focus light exiting the fiber into a small spot where a spark may form. Fibers of 0.7 and 1 mm diameter have been used with lengths of 1 and 2 meters. For some launch conditions sparks can (inadvertently) form at the fiber input, though this can be largely avoided by flowing helium gas at the input or by pulling vacuum. For 2 m length straight fibers of diameter 1 mm, the energy transmission was in the range of 80 to 90% [4]. Low launch angles (~0.02) were used to excite a minimum number of modes [45] allowing low angular divergence of light exiting the fiber and optimum exit beam quality of M²~15. With pulse energy of ~35 mJ the achievable focal intensity was ~470 GW/cm² well above the break down threshold intensity. Sparking at atmospheric pressure was achieved for 98% of laser shots, with the occasional misfires attributed to the varying multimode spatial profile (hot spots) in the exit beam. For straight configurations, the damage threshold of ~1 GHz/cm² is comparable to that for solid core fiber, and the main advantage of hollow core fibers lies in their improved output beam quality (smaller output angle) for a given core diameter. Bending loss studies showed that increased fiber bending reduced the energy transmission and reduced the beam quality at the fiber exit. For example, for 2-m length fibers with the first 1-m of straight, bending of radius of curvature (ROC) = 1.5 m yielded similar performance to the straight fiber, but with bending of ROC = 1 m sparking was no longer achievable at atmospheric pressure. Sparking at elevated pressure conditions is easier, for example at pressure of 14 bars, sparking was achieved with a 2-m fiber with ROC of 50 cm and bent fiber length of 1 m. Damage of the coated hollow fibers is generally due to optical damage of the reflective coating [43]. Varying the thickness and smoothness of the reflective coating can influence the damage threshold but there is a tradeoff with transmission efficiency and bend loss.

Despite the bending loss limitations, the coated hollow fibers (in relatively straight configurations) have been used for ignition of a single-cylinder of an inline 6-cylinder Waukesha VGF turbocharged natural-gas engine [44]. The engine has a nominal rating of 400 bhp at 1800 rpm with engine displacement of 18 liters. The focusing optics were integrated into an optical sparkplug which threaded into the sparkplug port of the engine cylinder and provided optical access to the cylinder through a sapphire window. The tests demonstrated 100% reliable ignition of the laser cylinder (with the remaining cylinders running on conventional spark ignition). The timing of the non-laser cylinders was kept at the original setting, nominally 14° before-top-dead-center (BTDC). The timing of the laser ignited cylinder was controlled independently, and retarded to 8° BTDC. Even with this delay, the peak pressure of the laser cylinder was reached before all other cylinders indicating an increased rate of heat release. Bihari and colleagues have also examined the use of coated hollow fibers for ignition using 532 nm radiation [46]. They were able to achieve spark formation at the fiber output using fibers of diameter 0.5, 0.7, and 1 mm. Using the hollow core fibers, the team also operated a Bombardier BSCRE-04 engine with the coupling set-up mounted such that it makes an angle of 15° with respect to the spark plug in the engine head.

4. Step-index silica fibers

Step-index silica fibers remain attractive for ignition applications owing to their low cost, versatility, and commercial maturity. As discussed above, the disparity in breakdown intensities of air (combusting gas mixture) relative to the fiber material requires that light exiting the fiber be demagnified by a factor of at least 10-20. This requirement has been very difficult to meet with conventional multimode (MM) silica fibers owing to the degraded beam quality (elevated M²) at the fiber output [23]. By paying close attention to the fiber launch and focusing optics, El-Rabii et al. have achieved a sparking rate of <1% in air at atmospheric pressure (increasing to 90% in air at 6 bar) with a demagnification of ≅10 using silica fiber with 940 µm core diameter [8]. The higher spark rate at elevated pressure is due to the pressure dependence of breakdown threshold. Previous attempts to use conventional multimode solid silica fibers for igniting engines have also been limited [47, 48]. Biruduganti et al. reported use of a 1 mm diameter core regular silica step index fiber to deliver 35 mJ of 532 nm light for igniting a single cylinder natural gas engine [48]. The published details of the optical setup used in their experiment are sparse, though they stated that they could not reliably spark in atmospheric pressure air or start the engine without altering the ignition timing. Mullett et al. used conventional 400 and 600 µm solid core step index silica fibers to operate an engine [47], but reported an ignition rate of only 35% while using 65 mJ pulse energy delivered through 600 µm core fiber, and 8% ignition rate with 50 mJ pulse energy delivered through 400 µm core fiber. They attributed the high misfire rate to poor beam quality at the fiber output.

Recent research has examined use of large clad MM fibers (clad-diameter to core-diameter exceeding ~1.1) for ignition applications owing to the possibility of improved exit beam quality (M²) [49–51]. Such fibers have been developed primarily for material processing applications where high average powers are needed and the large clad helps heat dissipation. While it remains critical to optimize the launch to minimize the number of modes excited at the input [8, 21, 52], the large clad approach additionally seeks to reduce the modal power diffusion as light propagates in the fiber. Characterization of several large core MM silica fibers (core sizes of 100, 200, and 400 µm) shows that larger clad dimensions provide lower mode coupling coefficients and higher output beam quality [50]. Mode coupling is attributed largely to curvature and imperfections at the core clad interface [53], analogous to micro-bending, and it is believed that such coupling is reduced by the increased mechanical rigidity of the large clad fibers leads [50, 54]. Figure 4 shows the possibility of high demagnification using the large clad fiber (1064 nm pulsed excitation) [49]. Indeed, reliable (100%) sparking was shown at the focused output of a large-clad fiber in atmospheric pressure air using 1064 nm light from a Q-switched Nd:YAG laser with pulse duration of 9.5 ns and input energy of 3.5 mJ (minimum). The commercial silica fiber (CeramOptec) had 400 µm core, 720 µm clad, and length 1.8 m. The fiber output could be focused to a diameter of 8 µm (demagnification of 50) giving a focal spot intensity of 420 GW/cm². The fiber output beam quality was M² = 2.5, which can be contrasted against a 400 µm core fiber with 440 µm clad having output M² = 38 and not allowing spark formation in atmospheric pressure air [55]. Onset of fiber damage occurred for input energy of 6-8 mJ, i.e., at (peak) core intensities of approximately 1 GW/cm². Extended pulse durations can be used as a means to increase the delivered pulse energy. With pulse durations of 50 ns, it was possible to deliver 25 mJ. Using 4 m long fibers, effects of fiber bending were examined for several bent and coiled configurations and reasonably good beam quality (M²<4) was observed for ROC of ~20 cm [56].

 

Fig. 4 Bottom: Plot of linear demagnifications, i.e., ratio of fiber core diameter to focused spot-size, achievable with large clad and regular fibers for low power 1064 nm excitation as a function of fiber core size (from [49]). Demagnification of >10-20 is required for spark formation of the fiber output in air. Plot also shows high power measurements from large clad fiber and El-Rabii et al. (minimum spot diameter of 100 mm for 940 μm core fiber) [8]. Top: Beam profiles. The left profile is from large clad fiber and the right from a regular clad fiber [8]. The former shows light largely concentrated into a single peak due to its higher spatial coherence (M² = 2.5), while the latter shows a speckle pattern typical of a multi-mode output.

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The large clad fibers were used for a single-cylinder engine demonstration on a Waukesha Co-operative Fuel Research (CFR) engine converted to run on bottled methane [51]. These tests, schematically shown in Fig. 5 , used a large clad fiber with length of 2.85 m, core diameter of 400 µm, and clad diameter of 720 µm through which 11 mJ pulses of 25 ns duration (M² = 5.1 and energy of 7 mJ at fiber exit). The final optical spark plug which focused the fiber output housed a diverging lens followed by a collimating lens and then a 10 mm focal length focusing lens (Gradium^TM). (The role of the diverging lens was to shorten the length of the overall plug by more strongly expanding the beam exiting the fiber). A 3 mm thick sapphire window with copper gaskets sealed the optical spark plug from high pressure combustion gases in the engine cylinder. The ignition timing was optimized for the electrical ignition system and the laser firing time was set to match that of the electric spark plug. The final output beam from the optical plug could form sparks in pressures as low as 3.5 bar which guaranteed sparking at higher pressures and allowed engine startup without changing the ignition timing. The fiber delivered laser ignition system allowed reliable spark formation and acquisition of combustion data [51]. A practical concern, which can limit spark formation, is the sensitivity of the fiber output beam quality to stresses from fiber positioning and mounting (owing to micro-bending beam quality degradation). More research is needed in this area as it was necessary to use low-stress mounting and adjust the fiber position to optimize the output.

 

Fig. 5 Experimental Setup for fiber delivered laser ignition (from [49]). (a) optic setup, (b) fiber configuration and path. 1) Laser, 2) Mirror, 3) Half Waveplate, 4) Polarizer, 5) Focusing Lens, 6) Fiber Holder, 7) Fiber, 8) Optical Spark Plug, and 9) Single Cylinder Engine.

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5. Photonic crystal and photonic bandgap fibers

In this section we summarize research on photonic crystal fibers (PCFs) and photonic bandgap fibers (PBGs) which employ periodic hole structures within the (silica) fiber material to modify the refractive index in such a way that one has efficient light guiding including single mode operation [57]. A PBG fiber with a 19 cell defect in the center was used to transmit ~0.5 mJ Nd:YAG pulses of duration 65 ns through a 2-m length fiber [58]. The authors believe higher pulse energies may be possible with improved mode matching. Tauer et al. have delivered pulses with energy of ~0.8 mJ and duration ~10 ns through a PBG fiber with 15 μm core-size and transmission of 82% [59]. Another researcher group transmitted picosecond pulse trains of 1064 nm light from an Nd:YAG laser through PBGs with inner core diameter of 13 microns. The maximum total energy that could be transmitted was approximately 1 mJ from a pulse train consisting of ~40 pulses each of 40 ps duration (25 μJ per pulse) [23, 60]. It was speculated that higher (total) pulse energies of ~100 mJ may be possible with a similar approach. Michaille et al. performed a study of high power transmission through both PBG and PCF fibers with comparison against a theoretical model [61]. The lower than expected damage threshold for the PBG fibers was attributed to laser-induced heating causing strain in the silica lattice. Ignition of rich methane-air mixtures at high pressures has been shown with 0.15 mJ transmitted through a hollow core PBG fiber [62], though conditions were favorable for ignition with low energy. The above results, while promising, generally have employed pulse energies less than is needed for practical applications. Owing to the inherent coupling between the single-mode output and the fiber dimension and geometry, increasing the transmitted pulse energies in PBGs and PCFs is challenging. Larger core (“rod-like”) PCFs may allow transmission of higher energies but if the fibers cannot be bent then they are not suitable for practical delivery [63]. Use of longer pulse durations may be of interest but damage and non-linear effects must be considered. Polymer fiber PBGs with one-dimensional Bragg gratings have also recently shown high optical output powers [64]

Finally, we highlight recent results with kagome hollow-core PCFs shown to deliver high pulse energies and ignite simple butane flames [65]. The kagome fibers feature much lower power overlap with the silica part of the fiber thereby allowing higher pulse energies while maintaining the single-mode output amenable to tight focusing [65]. Recent research has investigated two kagome fibers for high power delivery and ignition, one was a single-cell core with a cladding consisting of two rings of kagome lattice with a pitch of 28 μm and strut thickness of 640 nm, while the second was a 7-cell core with hypocycloid shape and three rings of kagome lattice cladding with a pitch of 20 μm and strut thickness of 320 nm – both fibers transmit light in the range ~700-1300 nm making them compatible with Nd:YAG (and similar) laser sources being considered for laser ignition. The maximum transmittable pulse energies were found to be in the range of 5-10 mJ for these fibers which is comparable to that required for laser ignition. In addition to running up against the silica damage threshold, the observed damage may also be related to contamination (dust) at the fiber input or imperfect launch conditions. The authors discuss that higher energies are likely possible with improved laser launch. As shown in Fig. 6 , ignition of simple butane flames was achieved at the focused fiber output with 2 mJ of delivered energy [65].

 

Fig. 6 Left: Experimental setup for the laser ignition with kagome fibers (from [65]). Right: Spark formation and ignition of butane fuel using output of kagome fiber (from [65]).

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6. Fiber lasers

Fiber laser systems are a rapidly progressing technology that may be useful as pump sources as well as direct ignition sources (cylinder-mounted or multiplexed). In addition to their potential for being power-efficient, compact, and inexpensive, the lasers inherently provide fiber delivery which can benefit versatile beam delivery and multiplexing. Pulsed systems can provide MW peak power and mJ energy pulses, i.e. parameters approaching those needed for laser ignition. Spark formation (in atmospheric pressure) air has been demonstrated with the focused output of a pulsed fiber laser [4]. These tests employed a multi-stage fiber amplifier system, seeded with electronically controlled nanosecond diode pulses, similar to the one described by Cheng et al. [66]; however, for spark formation, the last amplification stage used an 80-µm diameter core Yb-doped fiber yielding output beam quality of M² ~1.5. The architecture does employ some free-space coupling and components. The combination of large core and high beam quality is highly advantageous for achieving spark-generation with nanosecond-long and few-mJ energy pulses. Laser-breakdown in atmospheric pressure air was achieved using 0.7 ns duration fiber laser pulses with 2.4 mJ pulse energy and 3.4 MW peak power at 50 Hz repetition rate. For practical ignition systems one must consider the prospects for elevated pulse energies. The energy extraction in a fiber laser is ultimately limited by the bulk damage intensity threshold in fused silica, which scales (approximately) inversely proportionally to the square root of pulse duration. This behavior indicates that higher pulse energies in the range of 1-10 + mJ may be achieved by increasing the pulse duration, while maintaining sufficient focused intensities for spark formation [4].

7. Conclusions

Direct fiber delivery of high power pulses to form combustion initiating sparks remains a technical challenge. This short review has summarized needed fiber output parameters and research results using several types of fibers. Coated hollow core fibers have been used to deliver sparks in atmospheric air and for engine ignition. The primary difference relative to (conventional) solid silica fibers is the improved beam quality at the fiber exit, though the utility of the hollow fibers is limited by their performance degradation (beam quality and energy) due to bending. Earlier results with MM step-index silica fibers did not allow reliably spark formation and combustion ignition since the beam quality at the fiber exit was generally inadequate to achieve the needed focused intensities. More recent results with large-clad fibers, which provide improved mode quality due to reduced mode coupling, are promising though additional research is needed to improve the versatility of the approach and to mitigate mounting-induced stress effects. Nonetheless, reliable sparking in atmospheric pressure air and engine ignition have been demonstrated. Photonic crystal fibers (and photonic bandgap fibers) are attractive owing to their single mode output though deliverable energy is often limiting. Kagome fibers show particularly promising results. The increased complexity of such fibers should be considered as it impacts cost and durability for practical use. Spark formation has been demonstrated at the output of fiber lasers and, owing to the inherent fiber coupling and high efficiency, they may provide attractive laser ignition sources.

Recent progress with compact lasers may make them favorable for ignition applications due to their performance and cost. On the other hand, multiplexed fiber solutions that minimize the number of lasers (or gain elements) remain of interest. Such systems may also be able to leverage the laser sources advances for use in the full multiplexed system. The findings from high power fiber delivery can also benefit other areas of optical technology such as laser machining and diagnostics. For example, coated hollow core fibers have been used for combustion diagnostics of harsh environments with Coherent Anti-Stokes Raman Spectroscopy (CARS) [67]; similarly, the improved beam quality from large clad fibers has benefitted (ultraviolet) fiber delivered planar laser induced fluorescence (PLIF) [68, 69]. The present review has generally considered the use of non-resonant breakdown with nanosecond pulses. Of course, the use of other ignition schemes, for example involving thermal ignition, other wavelengths (resonant schemes), or multiple-pulse preionization [70, 71], that relax the fiber delivery requirements may dramatically change the landscape of laser ignition hardware systems.