High power fiber delivery for laser ignition applications Azer P. Yalin Department of Mechanical Engineering, Colorado State University, Fort Collins, CO, 80523, USA [email protected]
Abstract: The present contribution provides a concise review of high power fiber delivery research for laser ignition applications. The fiber delivery requirements are discussed in terms of exit energy, intensity, and beam quality. Past research using hollow core fibers, solid step-index fibers, and photonic crystal and bandgap fibers is summarized. Recent demonstrations of spark delivery using large clad step-index fibers and Kagome photonic bandgap fibers are highlighted. ©2013 Optical Society of America OCIS codes: (060.2270) Fiber characterization; (060.2310) Fiber optics; (060.2400) Fiber properties; (060.4005) Microstructured fibers.
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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-
#195031 - $15.00 USD Received 2 Aug 2013; revised 5 Sep 2013; accepted 6 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. S6 | DOI:10.1364/OE.21.0A1102 | OPTICS EXPRESS A1104
generation and gas compression, owing to the possibility of increased engine efficiency and reduced emissions. Figure 1 shows NOx 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 NOx reduces [7]. Laser ignition provides the lowest NOx 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.
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 powergeneration 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 sideand 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
#195031 - $15.00 USD Received 2 Aug 2013; revised 5 Sep 2013; accepted 6 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. S6 | DOI:10.1364/OE.21.0A1102 | OPTICS EXPRESS A1105
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 stepindex 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]).
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 peakpower (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., IBD,Air≅100300 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 #195031 - $15.00 USD Received 2 Aug 2013; revised 5 Sep 2013; accepted 6 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. S6 | DOI:10.1364/OE.21.0A1102 | OPTICS EXPRESS A1106
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]. 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
#195031 - $15.00 USD Received 2 Aug 2013; revised 5 Sep 2013; accepted 6 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. S6 | DOI:10.1364/OE.21.0A1102 | OPTICS EXPRESS A1107
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.
Fig. 3. Left: Schematic diagram of coated hollow fiber (from [21]). Right: Photograph of spark formation at output of coated hollow fiber (from [21]).
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 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.
Recent research has examined use of large clad MM fibers (clad-diameter to corediameter 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
#195031 - $15.00 USD Received 2 Aug 2013; revised 5 Sep 2013; accepted 6 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. S6 | DOI:10.1364/OE.21.0A1102 | OPTICS EXPRESS A1109
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
Abstract: The present contribution provides a concise review of high power fiber delivery research for laser ignition applications. The fiber delivery requirements are discussed in terms of exit energy, intensity, and beam quality. Past research using hollow core fibers, solid step-index fibers, and photonic crystal and bandgap fibers is summarized. Recent demonstrations of spark delivery using large clad step-index fibers and Kagome photonic bandgap fibers are highlighted. ©2013 Optical Society of America OCIS codes: (060.2270) Fiber characterization; (060.2310) Fiber optics; (060.2400) Fiber properties; (060.4005) Microstructured fibers.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
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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-
#195031 - $15.00 USD Received 2 Aug 2013; revised 5 Sep 2013; accepted 6 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. S6 | DOI:10.1364/OE.21.0A1102 | OPTICS EXPRESS A1104
generation and gas compression, owing to the possibility of increased engine efficiency and reduced emissions. Figure 1 shows NOx 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 NOx reduces [7]. Laser ignition provides the lowest NOx 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.
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 powergeneration 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 sideand 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
#195031 - $15.00 USD Received 2 Aug 2013; revised 5 Sep 2013; accepted 6 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. S6 | DOI:10.1364/OE.21.0A1102 | OPTICS EXPRESS A1105
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 stepindex 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]).
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 peakpower (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., IBD,Air≅100300 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 #195031 - $15.00 USD Received 2 Aug 2013; revised 5 Sep 2013; accepted 6 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. S6 | DOI:10.1364/OE.21.0A1102 | OPTICS EXPRESS A1106
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]. 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
#195031 - $15.00 USD Received 2 Aug 2013; revised 5 Sep 2013; accepted 6 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. S6 | DOI:10.1364/OE.21.0A1102 | OPTICS EXPRESS A1107
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.
Fig. 3. Left: Schematic diagram of coated hollow fiber (from [21]). Right: Photograph of spark formation at output of coated hollow fiber (from [21]).
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 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.
Recent research has examined use of large clad MM fibers (clad-diameter to corediameter 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
#195031 - $15.00 USD Received 2 Aug 2013; revised 5 Sep 2013; accepted 6 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. S6 | DOI:10.1364/OE.21.0A1102 | OPTICS EXPRESS A1109
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
