Movable fiber probe for gas-phase laser-induced breakdown spectroscopy

Cosmin E. Dumitrescu; Paulius V. Puzinauskas; Semih Olcmen

doi:10.1364/AO.47.000G88

Applied Optics
Vol. 47,
Issue 31,
pp. G88-G98
(2008)
•https://doi.org/10.1364/AO.47.000G88

Movable fiber probe for gas-phase laser- induced breakdown spectroscopy

Cosmin E. Dumitrescu, Paulius V. Puzinauskas, and Semih Olcmen

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Cosmin E. Dumitrescu, Paulius V. Puzinauskas, and Semih Olcmen, "Movable fiber probe for gas-phase laser-induced breakdown spectroscopy," Appl. Opt. 47, G88-G98 (2008)

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Abstract

A movable probe that fiber couples both the beam delivery and the signal collection functions of gas-phase laser-induced breakdown spectroscopy (LIBS) measurements was evaluated. The adjustable probe was used to investigate the effect of delivery fiber curvature on plasma characteristics and the associated effect on LIBS spectra and to further identify issues remaining to facilitate fully fiber-coupled gas-phase LIBS measurements. LIBS data were collected from lean methane–air mixtures of various equivalence ratios and spectroscopically analyzed to establish the ability to determine relative fuel–air ratio. Measurements with straight delivery fiber were compared to those with the fiber curved at specific radii. Decreasing fiber radius of curvature decreased fiber transmission efficiency and reduced the spark formation probability by almost a factor of 2. For constant fiber input energy, this decreased transmission increased the percentage of failed spark formations and influenced the LIBS elemental ratio calculations. However, minimal difference was found between LIBS measurements with straight or curved fiber as long as the output energy and a constant laser beam spot diameter were maintained on the exit beam focusing lens. A significant reduction in data scatter and improved linearity were achieved by using the Balmer series $H_{α}$ and $H_{β}$ hydrogen emission line ratio as a data selection criterion. Observed linear variation of $H / N$ elemental ratio with equivalence ratio confirmed the possibility of a flexible, light-contained, fully fiber-coupled probe for remote gas-phase LIBS analysis.

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Radius of Curvature (m)	Input energy (mJ)	Output Energy (mJ)	Fiber Transmission (%)	Total Laser Pulses	Total LIBS Sparks	LIBS Spark Probability (%)
Infinity (Straight)	68	35	51	3000	2297	77
	62	32.5	52	3000	2017	67
	59	31	53	3000	1920	64
	56	30	54	3000	1850	62
4	81	35	44	1000	796	80
3	73	32.5	44	1000	737	74
2	73	31	42	1000	678	68
1	81	30	38	1000	661	66

Fiber Radius of Curvature (m)	Distance between Fiber End and Collimating Lens (mm)	Beam Diameter in Front of Focusing Lens (mm)
Infinity (straight)	270	6.5
4	204	6.5
3	201	6.5
2	194	6.5
1	190	6.5

Radius of Curvature (m)	Energy (mJ)	Total LIBS Sparks	Unqualified $H_{656} / H_{486}$	Unqualified $H_{656} / N_{total}$	Unqualified $H_{656} / N_{746}$	Rejected Shots (%)
Infinity (straight)	35	2297	544	252	60	37.9
	32.5	2017	762	377	83	60.9
	31	1920	856	442	100	73.0
	30	1850	898	485	99	80.1
4	35	796	183	82	17	36.2
3	32.5	737	229	131	28	54.3
2	31	678	274	128	26	63.3
1	30	661	308	149	32	74.1

Fiber Output Energy and Radius of Curvature	$30 mJ$ $R = inf$	$31 mJ$ $R = \inf$	$32.5 mJ$ $R = \inf$	$34 mJ$ $R = \inf$	$35 mJ$ $R = \inf$	$36 mJ$ $R = \inf$	$30 mJ$ $R = 1 m$	$31 mJ$ $R = 2 m$	$32.5 mJ$ $R = 3 m$	$35 mJ$ $R = 4 m$
$R^{2}$	0.91	0.18	0.85	0.98	0.97	0.97	0.82	0.16	0.91	0.92
Φ	$H_{656} / H_{486}$ standard deviation
0.12	25.3	50.6	18.3	19.4	97.7	52.4	19.0	8.0	5.7	7.4
0.24	15.7	18.1	138.9	18.4	9.6	13.7	7.9	6.7	4.8	5.0
0.37	11.9	14.0	12.9	12.1	11.0	26.5	6.7	6.8	5.2	5.2
0.49	10.2	11.7	16.5	15.3	12.2	6.7	5.6	3.8	4.4	2.3
Φ	$H_{656} / N_{total}$ standard deviation
0.12	15.1	11.0	7.2	5.2	8.6	6.2	24.5	14.9	3.5	3.4
0.24	11.8	4.1	6.6	6.8	7.4	1.9	4.7	2.7	8.5	4.5
0.37	12.8	17.1	5.5	9.6	4.6	5.2	6.9	3.2	4.7	4.8
0.49	21.3	7.5	8.0	6.0	8.2	6.1	11.0	7.4	6.4	2.6

Fiber Output Energy (mJ)	30	31	32.5	34	35	36	30	31	32.5	35
Fiber radius of curvature (m)	Inf	Inf	Inf	Inf	Inf	Inf	1	2	3	4
Number of measurements	367	345	309	256	231	172	84	109	82	69
% total measurements	46	35	24	30	11	7	15	29	16	10

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Applied Optics