Methane-air mixtures ignited by CW laser-heated targets on optical 

fiber tips: Comparison of targets, optical fibers, and ignition delays 

Thomas H. Dubaniewicz Jr.1

National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory. P. O. Box 18070, 626 Cochrans Mill Road. 

Pittsburgh PA 15236. USA 

Tel.: + 14123866596; fax: + 14123866764. 

E-mail address: tcd5@cdc.gov.

Abstract 

Fiber optic systems are being deployed in locations where explosive gas atmospheres are normally present or are present under fault 

conditions. The National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory (NIOSH, PRL) conducted a study 

of laser safety in potentially flammable environments. Researchers conducted experiments to estimate the mean and standard deviation of 

laser powers needed 10 ignite 6% methane-air atmospheres using single mode optical fiber tips covered by two types of iron oxide (Fe3O4

and (FeMn)2O3) mixed with a ceramic adhesive. The iron oxides, heated by a 1064 μm continuous wave laser, ignited the methane-air 

mixtures at similar powers. The minimum igniting power and maximum non-igniting power (10 tests) were 407 and 350 mW, 

respectively, using a 62.5 μm fiber. Laser beams guided by 125 and 80 μm diameter cladding single mode fibers produced similar 

methane-air igniting powers, Ignition was not observed using coal particles at powers that produced ignition with the iron oxides. 

Threshold ignition delays using the single mode fiber were approximately proportional to the inverse square of the igniting power. 

Ignition delays were significantly longer than the reported activation time for a commercial fiber optic power limiter. Comparisons are 

made with the results of other researchers, 

1. Introduction 

Fiber optic systems are being deployed in locations where 

explosive gas atmospheres are normally present or are 

present under fault conditions. For example, fiber 

deployment through sewer systems and natural gas lines is 

occurring to overcome the "last mile" hurdle for metro fiber 

optic systems (Jeyapalan, 2002). Natural gas consists mostly 

of methane gas (typically greater than 85%), with variable 

and much smaller amounts of ethane, propane, other 

hydrocarbons, and hydrogen. Natural gas line faults produce 

flammable gas-air mixtures. Natural gas migrating from gas 

line faults into sewer systems has caused explosions. Sewer 

systems may contain flammable concentrations of 

methane-air generated from sewage. 

The optical radiation guided by fiber optic systems can, 

under certain conditions, be considered as a potential 

ignition source for explosive gas atmospheres. For 

wavelengths in the visible through mid-infrared spectrum, 

optical ignitions occur most easily when a target contacts the 

beam and converts the optical energy to thermal energy. For 

small beam diameters, the best targets for causing ignition 

absorb most of the optical radiation, do not vaporize as they 

become hot (i.e. do not dissipate thermal energy 

prematurely), and match the dimensions of the beam for 

maximum heat generation. An optical fiber tip that becomes 

dirty can form a target approximating these conditions. 

One method of preventing optical ignitions is to limit the 

beam power below levels needed to cause ignition. This 

method may be most advantageous where the laser beam 

may be exposed to a suitable target and flammable gas-air 

mixture under normal or fault conditions. For example, a 

fiber optic cable within a natural gas line that is ruptured 
Study Optical fiber core diameter (μm) Target material Methane-air (vol%) Minimum igniting power (mW) 

Wetzel et al. (2000) 62.5 Iron-manganese oxide layer 5 304

Zhang et al. (1992) 50 40 μm Appin coal particle 8 300

Table 1 

Minimum igniting optical powers and most easily ignited mixtures reported for methane-air 

accidentally. This single event is likely to produce a 

flammable gas-air mixture and an exposed optical fiber tip 

that becomes obstructed by foreign material (e.g. dirt). 

Optical equipment faults can produce over-power 

conditions that may pose an ignition risk. Independent 

over-power fault protection may be provided by optical 

devices spliced into the fiber optic system (Marrapode & 

Oron, 2003). These devices are designed for single mode 

fiber and require a certain amount of time to activate. 

Devices that activate before potential targets can heat 

sufficiently would prevent optical ignitions. The time that it 

takes to heat a target to cause ignition of the gas is referred 

to as the ignition delay. Detailed knowledge of methane 

ignition delays would be useful for designing power limiters 

capable of preventing ignitions. 

The National Institute for Occupational Safety and 

Health, Pittsburgh Research Laboratory (NIOSH, PRL) 

conducted a study of laser safety in potentially flammable 

environments.' Estimates of the mean and standard 

deviation of 1064 nm continuous wave (cw) laser powers 

needed to ignite 6% methane-air atmospheres using single 

mode optical fibers and several types of laser targets are 

reported. Ignition delay times are reported as a function of 

laser powers and compared to the activation time of a 

commercial optical power limiter. Comparisons are made 

with the results of other researchers. Results are applicable 

where methane gas poses an explosion hazard, such as the 

fiber-in-gas line and fiber-in-sewer installations. 

1The findings and conclusions in this report are those of the author and do 

not necessarily represent the views of the National Institute for Occupational 

Safety and Health. 

2. Literature review 

Minimum igniting powers for methane-air mixtures 

reported by Welzel, Schenk, Hau, Cammenga, and Bothe 

(2000) and Zhang, Hills, Zheng, Wall, and Samson (1992) 

are listed in Table 1. Target diameters above 62.5 μm

required higher powers for ignition than listed in the table. 

These results were obtained using a 1064 μm laser coupled 

to optical fibers, the laser beam guided to targets applied to 

the output tip of the optical fiber placed in the flammable 

atmosphere. Welzel et al.'s (2000) experiments were 

conducted in a vessel heated to 52°C. Not listed in the table 

are the results of Schenk, Bothe, and Cammenga (2001), 

who reported no ignitions of methane below 370 mW with 

mostly combustible targets attached to

62.5 μm core fiber tips. Minimum igniting powers for 

methane were not reported. The combustible targets 

included carbon black, anthracite coal, coking steam coal, 

high volatile coal, and a toner. The toner consisted mainly 

of ferrite powder on a styrene acrylate copolymer base. The 

'toner left a reddish residue on the fiber tip, presumably 

non-flammable ferric oxide (Fe2O3).

Several researchers reported lean methane-air mixtures, 

ranging from about 5% to 6% methane-air, produced the 

lowest igniting powers with cw lasers (Dubaniewicz, 

Cashdollar, Green, & Chaiken, 2000; Hawksworth, 2000; 

Welzel et al., 2000). One exception was Zhang et al, 

(1992), who reported 8% as the most easily ignited 

methane-air concentration. Hertzberg (1989) suggests lean 

methane-air mixtures ignite most easily because of 

selective diffusion. The lighter methane molecules enter 

the combustion zone faster than oxygen molecules, 

creating a fuel rich mixture in the ignition volume. Lean 

methane-air mixtures should therefore create most easily 

ignited stoichiometric mixtures (9.48 vol% (Kuchta, 

1985)) in the ignition volume close to the ignition source. 

Very short duration, high peak power pulsed ignition 

sources also ignite leaner methane-air mixtures most 

easily, but at concentrations greater than 6% methane-air. 

For example, 8.3% methane-air is used as the most easily 

ignitable concentration for mine apparatus approval testing 

with electrical sparks (MSHA. 1995). 6% methane-air was 

used for most ignition tests reported here. One series of 

tests was conducted using 8% methane-air in an attempt to 

duplicate Zhang et al.'s (1992) results. 

Threshold igniting laser powers generally decrease with 

beam diameter, but appear to level off for fiber-optic guided 

beam diameters below about 100 μm (Welzel et al., 2000). 

This leveling off has been factored into safe power 

recommendations for, optical systems used in flammable 

environments (ANSI/ISA. 2004; Carleton, Bothe, Proust, & 

Hawksworth, 2000a; McGeehin, 1994). Welzel et al, 

(2000) and Schenk et al. (2001) suggest the leveling off 

may be due to heat loss from' the fiber core to the fiber 

cladding, or may be due to a dimensional property of the 

heat source comparable to, but smaller than, the quenching 

distance of electrical sparks. The optical ignition power 

leveling off correlates with the common 125μm cladding 

diameter of the single mode and 62.5 μm fibers used for 

testing, adding credence to the cladding heat-sinking 

hypothesis. This suggests that fibers with smaller cladding 

diameters may produce ignitions with lower powers. In the 

current study, ignition tests were conducted with 

commercially available 
Although Moore and Weinberg (1987) did not use optical 

fibers, they observed that methane-air produced longer 

ignition delays than several other hydrocarbons tested, and 

ignition delays generally decreased hyperbolically as 

radiant power flux increased. Dubaniewicz et al. (2000) 

observed methane-air ignition delays increased with 

decreasing power, using coal and Fe3O4 targets on large 

core optical fiber tips, Ignition delays also increased with 

larger targets. 

3. Equipment 

An IPG Photonics PYL-10M Ytterbium fiber laser 

system was used for ignition experiments. The fiber laser 

wavelength was centered at 1064 nm and the output power 

was adjustable up to 10W cw. IPG indicated the overshoot 

was less than twice the set power, settling to the set power 

within 10 μs, The laser controller provided a laser-on 

electrical output signal used for ignition delay measurements. The system combines a red aiming laser with the 

high power beam within a single mode fiber optic pigtail. 

The pigtail was fusion spliced to a Corning single mode 1 

by 2 coupler to protect the laser from fiber fuses. Power 

levels were also kept below I W to avoid possible fiber 

fuses. A Scientec power meter (model D200PC) with 

attached calorimeter (model AC2500) measured the laser 

powers. Scientec checked the meter accuracy annually, 

traceable to National Institute of Standards and Technology 

(NIST) standards. Power measurements are reported to be 

accurate to within ±5%. 

Single mode fibers used for experiments included a 

9/125 (core/clad) urn diameter Coming SMF-28 silica 

fiber, and a 6.6/80 μm 3 M silica fiber. These single mode 

core diameters are the mode field diameters at the 1300 or 

1310 run wavelength. More than one mode propagates 

through these single mode fibers at the 1064 nm wavelength. Other fibers used for experiments included a 

62.5/125μm Corning multimode silica fiber and a 

1500/1650 μm Fiberguide Industries AFS silica fiber. The 

single mode and 62.5 μm fibers were fusion spliced to one 

of the couplers output ports. SMA connectors were used to 

couple the laser beam to the 1500 μm fiber. A fiber optic 

cleaver (York model FBK 11C) provided a flat, 

perpendicular, optical surface on the single mode and 62.5 

μm core fiber tips. A manual cleaving tool was used for the 

1500 μm fiber. 

Ignition experiments were conducted in a 20L test 

chamber (Fig. 1) designed for explosion testing of dusts, 

gases, and hybrid mixtures (Cashdollar & Hertzberg, 

1985). The chamber can be used at initial pressures that are 

below, at, or above atmospheric as long as the maximum 

explosion pressure is less than 21 bar, which is the rated 

pressure of the chamber. Sapphire windows allowed 

viewing inside the chamber. The concentration of the 

gas-air mixture was determined by partial pressures. The 

chamber pressure was set to 1 bar at room temperature. A 

fan within the chamber provided mixing. The chamber 

instrumentation included a pressure transducer 

80 μm cladding single mode fiber in an attempt to observe 

any influence of cladding diameter on igniting power. 

Lower observed igniting powers with the 80 μm clad fibers 

could justify lower safe power recommendations for 

optical beams guided by these fibers, 

Optical equipment may be installed in locations where 

potential beam targets are not well known or controlled. For 

these situations, researchers have taken a worst-case 

approach in selecting targets for experimental research. 

Using cw lasers with small beam diameters, strongly 

absorbing and thermally stable targets appear to ignite 

flammable gases most easily. Moore and Weinberg (1987) 

were unable to ignite methane-air mixtures using laserheated cotton wool particles, which turned to ash and 

disintegrated. They were able to ignite methane-air 

mixtures using thermally stable refractory insulating wool 

as the laser target. The insulating wool absorbed the 

selected laser wavelength well. Welzel, Bothe, and 

Cammenga (1994) found black iron manganese oxide 

(FeMn)2O3, targets produced the lowest igniting powers of 

a number of targets tested under most experimental conditions using cw lasers. Schenk et al. (2001) found that a toner 

consisting mostly of ferrite powder ignited certain other 

flammable atmospheres most easily, but not at powers 

below that obtained with the (FeMn)2O3 targets. 

Dubaniewicz et al. (2000) found Fe3O4 targets produced 

slightly lower igniting powers than coal targets in 

methane-air mixtures, using a non-flammable grease' to 

attach the targets to large core optical fibers. Larger coal 

particles required higher laser powers to ignite the 

methane-air, suggesting the heat of combustion from the 

coal was not a significant contributor to ignition compared 

to the heat generated by the laser. Hawksworth (2000) 

observed combustible 'targets burn away without igniting 

selected gases at power levels that produced ignitions with 

(FeMn)2O3. The (FeMn)2O3 produced lower igniting 

powers for selected atmospheres when it was mixed with a 

ceramic adhesive and applied to large core diameter 

rnultirnode optical fiber tips as a paste (Carleton, Bothe, 

Proust, & Hawksworth, 2000b). One exception was Zhang 

et al. (1992), who reported that Appin coal particles ignited 

methane at the lowest powers. 

The (FeMn2)O3 and Fe3O4 absorb well over a range of 

wavelengths (as reported in the materials section), and so 

can be heated efficiently with a variety of laser wavelengths. 

These iron oxides are not 'flammable, and so do not vaporize 

or disintegrate easily upon heating. Mixing the iron oxides 

with a ceramic adhesive should enhance the thermal stability 

of the laser target. The adhesive should also reduce the 

likelihood of the target dislodging from the fiber tip during 

tests. Coal and combinations of iron oxide and ceramic 

adhesive mixtures were used as targets in the current study 

as a conservative worst-case approach to targets that may 

obstruct optical fibers. 

Zhang et al. (1992) reported a 45 ms delay time for 

igniting 8% methane-air using 360 mW to heat a 40 μm 

Appin coal particle attached to a 100 μm core fiber tip. 
Fig. l. A 20 L chamber laser ignition test setup.

(Viatran model 218-28). A persona] computer (PC) based 

data acquisition program recorded the pressure transducer 

signal and the laser-on signal from the laser controller. A 

high-speed video camera (NAC model 400) recorded some 

of the tests. 

4. Target materials 

Two types of iron oxide were used as laser target 

materials. Bayferrox 303 T is a mixed iron manganese oxide 

((FeMn)2O3) with a 0.6 μm particle size. Black iron oxide is 

a combination of ferrous (FeO) and ferric (Fe2O3) oxide 

having a theoretical formula of Fe3O4 and average particle 

size of 0.4 μm. These iron oxides are non-flammable. 

The coal dust was Pittsburgh seam, high volatile 

bituminous coal mined from the NIOSH PRL Safety 

Research Coal Mine. The volatility is 37% and ash content 

is 6% (typical) (Cashdollar, 1996). The coal dust was sieved 

to produce 38-45 μm particles.

Labsphere, Inc. measured reflectance factors for the target 

materials over a spectral range from the ultraviolet to 2500 

nm (Fig. 2). In Fig. 2, the black iron oxide becomes a 

stronger absorber than Bayferrox 303 T for wavelengths 

above 950 nm. At the 1064 μm wavelength, the absorptance 

(1-reflectance factor) of the Bayferrox 303T and coal was 

85%, and the absorptance of the black iron oxide was 92%. 

Other researchers reported higher coal dust absorption at the 

1064 nm wavelength (Schenk. et al., 2001). The coal dust 

sample sent to Labsphere consisted of particles significantly 

larger than the iron oxide samples, and reflecting facets 

could be seen on many coal particles under the microscope, 

probably increasing the reflectivity 

Fig. 2 Reflectance of Pittsburgh coal black iron oxide and Bayferrox 303T 

from 350 to 2500 nm. 

of the coal sample relative to the iron oxides. All three 

materials absorb well at the 1064 nm wavelength. 

For most tests the iron oxides were mixed with a Zirconia 

based ceramic adhesive (Cotronics Resbond 904), 20% iron 

oxide to 80% adhesive by weight. The white adhesi.ve 

mixed with the iron oxides produced a gray colored mixture, 

likely absorbing the laser less efficiently than the iron 

oxides alone. The Resbond 904 provides adhesion up to 

2200°C. 

5. Methods 

An adequate length of fiber was pulled through a feedthrough in the 20 L chamber (Fig. 1) to allow preparation of 

the fiber tip. A fiber optic cleaver (York model FBK 11 C) 

provided a flat, perpendicular, optical surface for each 

ignition test. The power emanating from the cleaved end of 

the fiber was measured before and after the test. For tests 

with the Resbond 904 adhesive, a thin layer of the target 

paste sufficient to block the red aiming laser was applied to 

the fiber tip and air dried with the aid of the mixing fan. For 

tests with no adhesive, the (FeMn)2O3 was mixed with ethyl 

alcohol and applied to the fiber tip as a thin layer until the 

aiming laser was blocked. The (FeMnh03 alcohol mixture 

was air dried before proceeding, presumably leaving only 

the (FeMn)2O3 on the fiber tip. For the coal dust 

experiments, the particles were placed on the fiber tip by 

pushing the fiber along a dish containing the particles. 

Blockage of the aiming laser indicated the coal covered the 

fiber tip. Single mode fiber was used for the coal particle 

tests in an attempt to ensure that most of the laser beam was 

absorbed by the particles. 

The fiber was placed in the chamber pointing upward. 

The chamber was evacuated, and methane-air concentrations were' set using partial pressures. The mixture was 

allowed to settle for about I min after fan mixing. For tests 

with no adhesives, researchers looked through the window 

at the top of the chamber to ensure the aiming laser was 
not visible, indicating the target was still attached after 

mixing. Ignitions produced over-pressures of greater than 

3.3 bar and non-ignitions produced over-pressures of less 

than 0.05 bar. Non-ignitions were generally repeated 10 

times before ending a test series (Table 2). 

Table 2 

Methane-air threshold igniting powers 

Optical fiber core/ 

clad (μm) 

Target Methane-air 

(%vol)

Threshold igniting 

power (W/mm2) 

Igniting power density 

(mW)

Maximum non-igniting 

power (10 tests) (mW) 

9/125 Pittsburgh coal 8 — — 620

9/125 (FeMn)2O3 6 518 5180 470

6.6/80 (FeMn)2O3-Resbond 904 6 431 12600 380

9/125 Fe3O4-Resbond 904 6 417 6560 380

62.5/125 (FeMn)2O3-Resbond 904 6 407 133 350

1500/1650 Fe3O4-Resbond 904 6 5000 2.83 4500

The ignition delays were measured from the PC data plots of 

the laser-on signals, pressure traces, and rate of pressure rise 

(dP/dt) traces versus time. High speed video recordings of 

selected tests showed the flame front emanating from the 

fiber tip about 56 ms before an appreciable rate of pressure 

rise in this chamber. Ignition delay times reported here based 

on the pressure transducer measurements were reduced by 

56 ms to account for the difference. Microsoft Excel 2002 

was used to calculate data trends and coefficients of 

determination (R2). 

Researchers used the Bruceton Up and Down method 

(Dixon & Massey, 1983, Chapter 19; Dixon & Mood, 1948) 

to estimate the igniting power mean and standard deviation. 

The mean igniting power is the power level that has a 50% 

probability of igniting the gas. An initial laser power was 

chosen for an ignition test. If the test resulted in an ignition, 

the laser power was decreased by 50mW for the next test. 

The laser power was increased by 50mW for a subsequent 

test when a test resulted in a non-ignition. After each series 

of Up and Down tests were completed, the number of 

ignitions and non-ignitions were counted separately, and the 

group that produced the lower tally was used for the 

statistical analysis. 

The Up and Down method requires several experimental 

conditions be met, and these conditions may not be known 

beforehand. The method requires the variate to be 

approximately normal near the mean. Prior to testing, 

researchers chose to use the igniting power itself as the 

variate. Shapiro-Wilk tests, normal probability plots, and 

boxplots were used to assess the distributions. The single 

mode fiber distributions were accepted as being approximately normal near the mean. The Up and Down method 

also requires the testing step to be less than 2 standard 

deviations to simplify the statistical analysis. Researchers 

chose 50 mW as the step power prior to testing, and 

subsequently this choice was also accepted as satisfying the 

method requirements. 

6. Results 

Table 2 summarizes results of tests to determine which 

targets produced the lowest methane-air igniting powers. 

Threshold igniting power is the lowest igniting power 

observed under specified test conditions. The tests using 

Pittsburgh seam coal and no adhesive (Table 2, row 1) were 

an attempt to duplicate Zhang's reported 300 m W ignition 

(Table 1), but with single mode fiber and Pittsburgh coal, 

No ignition was observed in 10 attempts using 310 mW. The 

power was doubled to 620 mW and, again, no ignition was 

observed in 10 attempts. The coal particles burned off or fell 

off the tip during heating without igniting the gas, The 5 W 

threshold igniting power for the 1 500/1650 μm fiber tip 

blocked by the iron oxide-ceramic adhesive mixture is the 

same as that reported by Hawksworth (2000), The iron 

oxides mixed with the ceramic adhesive ignited the 

methane-air at lower powers than with no adhesive, in 

agreement with results reported by Carleton et al. (2000b). 

The iron oxide-ceramic adhesive mixtures were therefore 

selected as a conservative worst case target for the more 

extensive Up and Down tests. 

Table 3 lists mean igniting powers and standard 

deviations obtained using the Up and Down method. The 

laser heated the targets to incandescence in all tests, 

Table 3 

Up and Down test results for igniting 6% methane-air mixtures

Optical fiber core/ 

clad (urn) 

Target n Mean igniting power 

(mW)

Standard deviation 

(mW)

9/125 Fe3O4-Resbond 904 23 535 127

9/125 (FeMn)2O3-Resbond 904 24 498 80

9/125 Combined iron oxide series 47 516 113

6.6/80 (FeMn)2O3-Resbond 904 40 521 115
Fig. 3. A 6% methane-air ignition delay vs. laser power using Fe3O4 and 

(FeMn)2O3 targets attached to single mode fiber tips with or without a 

ceramic adhesive. Curve fit to threshold delay (r) vs. igniting power (P), 

and coefficient of determination (R2) also shown. 

Ignition or no ignition. For many non-ignitions, the 

incandescence ceased or decreased significantly within a 

few seconds. The aiming laser was visible after many of 

these non-ignitions, indicating the target was no longer 

attached or partially attached. As reported in the Discussion 

section, a T-test indicated the Fe3O4 and (FeMn)2O3

probably produce similar mean igniting powers. Based on 

this assumption, and being able to combine the 9/125 data to 

form one continuous Up and Down series, the mean and 

standard deviation of the combined iron oxide data were also 

listed in Table 3, 

The Up and Down tests provided most of the ignition 

delay data. Fig. 3 shows ignition delays obtained using the 

single mode fiber and all targets. Ignition delays using the 

single mode fibers were quite variable, ranging from less 

than 0.1 s to over 30 s. Fig. 3 shows only data, for ignitions 

that occurred before 1 s. An approximately inverse square 

curve fit reasonably well (R2> 0.96) through the shortest 

ignition delay (t) measured at each power level (P). These 

threshold data are listed in Table 4, Zhang et al.'s (1992) 

45 ms delay for igniting 8% methane-air using 360 mW to

heat a 40 μm particle on a 100 μm fiber is also shown in 

Fig. 3. 

Table 4 

Threshold igniting delay times using single mode fiber

Fiber core/clad (μm) Target Laser power (mW) Threshold delay (s) 

9/125 Fe3O4-Resbond 904 417 0.340

9/125 Fe3O4-Resbond 904 475 0.272

6.6/80 (FeMn)2O3-Resbond 904 527 0.199

6.6/80 (FeMn)2O3-Resbond 904 580 0.160

9/125 (FeMn)2O3 618 0.118

6.6/80 (FeMn)2O3-Resbond 904 664 0.111

91/125 Fe3O4-Resbond 904 808 0.086

Zhang et al.'s (1992) reported 45 ms delay is much 

shorter than the delay times reported here (Fig. 3 and Table 

4). Some aspects of Zhang et al.'s (1992) result are more 

indicative of high peak power pulsed ignition possibly 

caused by excessive laser overshoot. The maximum laser 

power setting ·was required to produce methane-air 

ignitions in Zhang et al.’s (992) study. This, along with the 

use of external focusing optics, may have caused significant 

back reflection. A strong back reflection can cause certain 

lasers to become wildly unstable (Force Inc., 2005). The 

surface absorbing calorimeter used by Zhang et al. (1992) 

may not have had a sufficiently fast response time to detect 

excessive overshoot. 

Zhang et al. (1992) found 8% methane-air to be the 

optimum concentration using the cw laser. This result is 

more indicative of high peak power pulsed ignitions. Zhang 

et al. (1992) also found 8% methane-air to be an optimum 

concentration using a pulsed laser for additional experiments. The 8% optimum concentration is similar to that 

used for mine apparatus approval testing with electrical 

sparks (MSHA, 1995). Other researchers observed lower 

optimum methane-air concentrations using cw lasers 

(Dubaniewicz et al., 2000; Hawksworth, 2000; Welzel et 

al., 2000). Perhaps ignition occurs at 8% most easily with 

the short duration sparks because diffusion effects described 

by Hertzberg (1989) don't have enough time to create 

stoichiometric concentrations with leaner mixtures before 

the ignition source is removed. 

Zhang et al.'s (1992) 45ms result was obtained using a

22% volatile matter content coal particle target. Schenk et

al. (2001) found that high volatile coal targets produced the 

highest igniting powers and longest ignition delays among 

a number of targets tested for selected flammable atmospheres other than methane. Schenk et al. (2001) did not 

observe ignition of methane-air using powers up to 

370 mW with any target. Prior research (Dubaniewicz et 

al., 2000) suggests heat of combustion contributions 

from the small coal particle should be relatively small 

compared to the heat generated by the laser, and should 

not have such a significant impact on ignition delay. 

Excessive laser overshoot could produce apparently lower 

igniting powers, shorter ignition delays, and richer 
concentrations of gas that ignite most easily, which is more 

consistent with pulsed ignition sources. 

Another conceivable explanation could be that a smaller 

target produced the 45 ms ignition. If it could absorb most 

of the beam, a smaller target would heat more quickly and 

attain higher temperatures than a larger target. Zhang's 

particle conceivably could have partially disintegrated prior 

to ignition, producing a smaller target. Differences in 

observed ignition delay times are significant enough to 

warrant independent verification. 

The approximate inverse square relationship between 

threshold ignition delay and power (Fig. 3) is similar to the 

hyperbolic relationship observed by Moore and Weinberg 

(1987). One commercial optical power limiter designed to 

prevent excessive powers in single mode fiber activates 

within 5 μs under certain test conditions (Marrapode & 

Oron, 2003). Activation time varies with power. This 

activation time is significantly less than the ignition delays 

shown in Fig. 3, listed in Table 4, and reported by Zhang et 

al. (1992). The power limiting devices may prove to be 

effective for methane-air ignition prevention. Ignition tests 

with the devices should be conducted to verify ignition 

prevention before they are used for such purposes. 

A T-test for unequal variances was used to compare the 

mean igniting power of the 9/125 Fe3O4 Resbond 904 and 

9/125 (FeMn)2O3 Resbond 904 targets (Table 3). The T-test 

indicated the mean igniting powers could be considered 

similar at a 0.1 significance level. Any difference in mean 

igniting power using the two iron oxides was not detected. 

Using the T-test with appropriate assumptions for equal or 

unequal variances, the 6.6/80 (FeMnh03-Resbond 904 

mean igniting power could be considered similar to any of 

the mean igniting powers for the 9/125 Resbond 904 data 

(Fe3O4 (FeMn)2O3 or combined iron oxides) listed in Table 

3 at a 0.1 significance level. The mean and standard 

deviation of the combined 9/125 iron oxide-Resbond 904 

series appears to converge with the values for the 6.6(80 

(FeMn)2O3 Resbond 904 series. Any heat sinking effects of 

the cladding material on average igniting powers were not 

detected. The different types of iron oxide or cladding 

diameters do not appear to affect igniting powers 

significantly. 

Minimum igniting powers reported by Welzel and Zhang 

(Table 1) were lower than minimum igniting powers 

observed in this study (Table 2). The inability to ignite 

methane at the lower powers or with flammable targets in 

this study is more in line with Schenk et al.'s (2001) and 

Moore and Weinbergs's (1987) results. Excessive laser 

overshoot could produce apparently lower igniting powers 

than reported here. If it could absorb most of the beam, a 

smaller target than used in this study would attain a higher 

temperature and conceivably ignite methane at a lower 

power, Alternately, the up and down tests suggests 300 mW 

may be several standard deviations from the mean igniting 

power, and so more attempts may produce ignition using 

300 mW. 

Other potential ignition mechanisms should be considered as appropriate. For example, Hawksworth (2001) 

reported 300 mW readily caused smoldering ignition in 

combustible coal dust layers, which could possibly lead to 

large scale burning and subsequent ignition of flammable 

gas. Power thresholds derived from tests of targets 

obscuring optical fiber tips suspended in flammable gas 

atmospheres such as studied here may not be conservative 

for all conceivable ignition mechanisms, 

Heavier hydrocarbons generally ignite at lower powers 

and more quickly than methane (Moore & Weinberg, 

1987). Additional research is warranted to determine 

ignition delay times of heavier hydrocarbons (e.g., propane) 

for natural gas that may contain significant amounts of 

heavier hydrocarbons. 

8. Conclusions 

For 6% methane-air mixtures ignited by laser heated iron 

oxide-ceramic targets on single mode fibers tips, the mean 

igniting power was about 520 mW with a standard 

deviation of about 110 mW. The igniting power distribution 

near the mean was approximately normal. The 62.5 μm

fiber produced the minimum igniting power (407 mW). No 

ignition was observed with the 62.5 μm fiber in 10 tests 

using 350 mW. 

Fe3O4 and (FeMn)2O3 targets mixed with a ceramic 

adhesive and heated by a 1064 nm laser ignited methane-air 

at similar powers. Ignition was not observed using coal 

particles without adhesive at powers that produced ignition 

with the iron oxides. 

. Using single mode fibers, threshold ignition delays were 

approximately proportional to the inverse square of the 

igniting power. Ignition delays were significantly longer 

than the reported activation time for a commercial fiber 

optic power limiting device, suggesting such a device may 

be useful for over power fault protection to prevent 

methane-air ignition. 

Laser beams guided by 125 and 80 μm diameter cladding 

single mode fibers produced similar mean ignition powers. 

Heat sinking effects of the cladding material were not 

observed. The leveling off of threshold powers below the 

62.5 μm beam diameter may be comparable to, but smaller 

than, the quenching distance of electrical sparks. More 

conservative safe power recommendations for beams 

guided by 80 urn diameter cladding single mode fibers are 

not warranted based on these results. 

Acknowledgments 

The author thanks Kenneth L Cashdollar, NIOSH, PRL, 

for guidance through all phases of this work. The author 

also thanks Linda McWilliarns, NIOSH, PRL, for help with 

the statistical analysis. Mention of company names or 

products does not constitute endorsement by the National 

Institute for Occupational Safety and Health. 
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