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A new compact laser source for portable LIBS applications

J. Goujon1, O. Musset1, A. Giakoumaki², V. Pinon², D. Anglos², E. Georgiou3

1 Institut Carnot de Bourgogne, UMR 5209 CNRS-Université de Bourgogne, 9 Av. A. Savary,

BP 47 870, F-21078, DIJON, Cedex, France 2 Institute of Electronic Structure and Laser, Foundation for Research and Technology Hellas

(IESL-FORTH), Heraklion, Crete, Greece 3 Department of Electrical Engineering, Technological Educational Institute of Crete, GR 71004

Heraklion, Crete, Greece

Abstract

We present LIBS experimental results that demonstrate the use of a newly compact, versatile pulsed laser source in

material analysis in view of research aiming at the development of portable LIBS instrumentation. LIBS qualitative

analyses were performed on various samples and objects, and spectra were recorded in gated and non-gated modes. The

latter is important because of advantages arising from size and cost reduction when using simple, compact spectrograph-

CCD detection systems over the standard ICCD-based configurations. The new Nd3+:YAG laser source exhibited very

reliable performance in terms of laser pulse repeatability, autonomy and interface. Indeed, it can deliver a 45 mJ for 4.5 ns

pulse and work at 1 Hz. Having the ability to work in double-pulse mode, it provided versatility in the measurements

leading to increased LIBS signal intensities, improved the signal noise ratio and stabilized spectra. The first test results are

encouraging and demonstrate that this new laser is suitable for integration in compact, portable and low cost LIBS sensors

with a wide spectrum of materials analysis applications.

Keywords

LIBS, Nd3+:YAG, Q-switched laser, double pulse, portable, low cost, NimH batteries, USB PC control, non-gated

spectrometer, microcontroller, tripling frequency

1 Introduction

Laser-Induced Breakdown Spectroscopy [1, 2] is a technique of analysis of the chemical composition of a material, which

presents a set of particularly attractive characteristics: simultaneous multielementary analysis, applicable to any type of

materials (solid, liquid, gas, spray) real time, at distance. The principle of the LIBS technique consists in focusing an

impulsive laser beam on the material to be analyzed to create a very warm microplasma from a small quantity of ejected

Solid State Lasers XVII: Technology and Devicesedited by W. Andrew Clarkson, Norman Hodgson, Ramesh K. Shori

Proc. of SPIE Vol. 6871, 68712Q, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.777953

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matter. The spectrum analysis of the light emitted by the plasma allows to determine the nature and the concentration of

the various chemical elements which compose the material.

LIBS has been gaining recognition as a highly versatile spectrochemical method for materials analysis owing to its

relatively straightforward implementation that makes it adaptable to a diverse range of demanding analytical problems in

industrial process control, environmental monitoring, remote sensing, [3, 4, 5] security applications and archaeological

science or art conservation [6, 7]. Considerable attention has been devoted in recent years to the development of compact-

size, integrated field-deployable LIBS systems because of the needs for rapid, multi-elemental analysis and identification

or classification of materials, on site. In parallel, several transportable or portable LIBS spectrometers have also appeared

in the market over the past few years advertising LIBS technology to a broader community of users. One of the first

compact LIBS prototypes was developed at Los Alamos for the analysis of beryllium [8].

Despite impressive technological advances in laser sources [micro-chip LIBS] and detectors, compromises need to be

made in order to obtain a truly portable instrument. For example, small but inevitably less powerful lasers are chosen,

which often offer low or no control on their output energy. On the spectrometer end, small-size monochromators that offer

very good spectral resolution and relatively broad spectral coverage are available but they are equipped with non-

intensified CCD array detectors that limit flexibility in the time-resolved recording of the spectra even though some of

them do give some control on the start time of the detection window. Obviously, depending upon the analytical

application and the technology available, appropriate choices are made to satisfy in a least compromising way the overall

system performance.

In the present work we investigate the performance of a new compact, low-cost, flash-lamp pumped, pulsed Nd3+:YAG

laser operating at 1064 nm, which we are planning to include in a portable LIBS prototype. The laser is controlled through

a microcontroller (with USB connection to PC) and is battery powered offering a great degree of autonomy to the LIBS

spectrometer. Furthermore, unlike lasers of similar size, which are usually passively Q-switched by a saturable absorber,

the one described in this study is based on an active electro-optic Q-switch that offers an excellent control on the pulse

energy and accurate trigger timing. In addition the laser has the option of delivering pairs of pulses, typically separated in

time by several tens of microseconds, being appropriate for double-pulse (DP) LIBS measurements.

Recent studies show that DP-LIBS yields improved analytical performance, featuring enhanced emission intensity along

with lower detection limits for a wide range of materials [9, 10]. The observed emission signal enhancement in DP-LIBS

analysis of solids in ambient air is attributed to the fact that the ablation plume arising from the second laser pulse

experiences a locally modified atmosphere over the sample surface, which is a result of the plume expansion following

irradiation with the first laser pulse. Obviously the dual-pulse option available by this compact laser provides a clear

advantage to the proposed compact LIBS spectrometer, which will be capable of supporting both single and double pulse

LIBS measurements increasing analytical sensitivity and versatility. In this paper we present technical specifications of the

laser and its output characteristics and concentrate on the evaluation of its performance with respect to LIBS analysis of

various solids. Measurements were carried out both with a standard monochromator ICCD detector system as well as with

a small size CCD array spectrometer and have resulted in encouraging results regarding the capabilities of the new

compact laser with respect to portable LIBS instrumentation.



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2 LIBS Laser source

2.1 Laser characteristics and operation

A novel compact and portable pulsed laser system was developed in order to meet the size, weight and performance

requirements for in-situ LIBS diagnostic applications. Additionally, the enhanced capabilities and output characteristics of

this laser like high energy stability, short pulse duration, single or dual pulse output, accurate pulse timing and ns trigger

synchronization render it suitable for testing and advancing novel state of the art LIBS measurement techniques, as well as

improving the quantitative LIBS result precision and reliability, as it will be described later.

The laser system is composed of a laser head incorporating its associated electronic drivers, and a “control unit”, which

integrates a microcontroller card controlling the whole system plus a human-interface electronics (screen, buttons, trigger

connections…) as it is shown below in Fig. 1a. Besides, the laser could be managed by a Visual Basic homemade PC

software (Win LIBS) via an USB connection towards the microprocessor (Fig. 1b).

Approximate dimensions of the cylindrical laser head (Fig. 1c) are 12 cm length x 5 cm diameter, its weight less than

0.5 Kg while the weight of the whole system (head and control unit) presently does not exceed 3 Kg, including a built-in

pack of rechargeable batteries capable of maintaining the system in operation for up to 10 hours while delivering several

hundreds of laser shots. A high efficiency electronic circuit using low-cost components, developed at ICB, is employed to

drive the laser and allow 1 Hz operation.

Figure 1 : a) Overall view of the compact laser system for advanced LIBS diagnostics ; b) WinLIBS software ; c) 3D laser head

+ its power supply + its focusing system

In spite of large progress in laser development and miniaturization over the last 3 decades, to-date there exist very few

compact commercial laser systems suitable for portable LIBS applications and even fewer of those models possess

adequate features and advanced capabilities for demanding LIBS diagnostics. These are mainly based on a saturable

absorber passive Q-switch. In principle though, this simple and cost effective configuration would imply that these lasers

might present issues with pulse energy adjustability and stability, but also problems of inadequate control of spectrometer-

detector triggering. In our present work, the development of a novel system of equivalent size and low cost was motivated

by the advantages of employing active Q-switching, improving output performance and integrating rechargeable battery



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electrical supply, yielding true portability and autonomy of several hours (depending of course on the pumping level and

the shots frequency) for field applications.

The laser source is a Nd3+:YAG crystal based cavity, pumped by a mini flashlamp and actively Q-switched with a mini

Pockels cell. The maximum energy delivered by the laser is about 45 mJ in a single Q-switched pulse with a FWHM (Full

Width at Half Maximum) of 4.5 ns. Alternatively, by repeated Q-switch cycling at specific instants, two (or more) output

pulses, again of ns-duration, with controlled timing separation and predictable energies can be extracted from the

resonator, useful for running dual-pulse LIBS diagnostic techniques with an uncomplicated laser source. The single pulse

output energy as function of flashlamp electrical input is given in Fig. 2a, while the pulse duration plotted as function of

output energy (Fig. 2b) shows that pulse durations are shorter than 9 ns for all output energies of practical interest.

Pulse durations below 5 ns can easily obtained from this system and maintained constant for any output energy within its

specifications, by operating the laser at the high energy regime and employing an optical attenuator at the output for pulse-

energy adjustment. Such short durations can be important for efficient and repeatable LIBS signal obtained over a large

range of laser energies as required by the material properties of the sample. In this respect, good shot to shot pulse-

duration stability and pulse energy repeatability are both very important parameters for reliable LIBS measurements. This

is actually the case with our laser as it is demonstrated in Fig. 2c with a superposition of 50 successive pulses. Indeed, for

a mean pulsewidth value of 5.57 ns, the dispersion of the FWHM is only 0.12 ns.

Figure 2 : a) Energy and b) Pulsewidth (FWHM) of the Q-switched laser pulse ; c) laser pulse stability, demonstrated by

superposition of 50 pulses.

Furthermore, because of the improved laser technology and materials and the low pumping energies used, no heating

problem is present and the system can work as long as desired with the same few percentage of stability even though it is

only air-cooled.

The laser beam diameter is about 3 mm with a uniform top-hat intensity profile, while its M² factor of quality has been

measured at approximately 30. Although these parameters indicate that the laser beam cannot be focused very tightly, on

the other hand it will usually guarantee a uniformly representative sampling in several cases of LIBS measurements on

inhomogeneous or grainy materials (ceramics, biological, etc…). Further laser re-design work is planned to improve beam

characteristics, so that smaller focal spot sizes can be obtained if so desired, though all tests performed so far and results



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presented in this work indicate that the beam-focus ability of the current system meets the practical requirements for all

LIBS applications investigated to date.

As mentioned above, the system is also capable to deliver several laser pulses in one lamp flash. Indeed, because of the

fast high-voltage switch used to drive the Pockels cell, it is possible to extract several separate laser pulses during the

flashlamp duration, the only limit being imposed by the laser population dynamics. Specifically, a minimum time interval

of few tens of microseconds between two pulses is necessary to guarantee sufficient population inversion build-up for an

energetic second pulse, considering the preceding depletion. Thus, by double Q-switching, two laser output pulses are

generated during the same pump lamp flash (Fig. 3a). The output energy of the two pulses can be adjusted by varying two

timing parameters, namely the delay of the first pulse relative to the flashlamp trigger and the additional delay of the

second pulse relative to the first. These delays are adjusted to be sufficiently long (several 10's of microseconds each) to

allow adequate population inversion buildup in the laser crystal for each pulse separately. The duration of the above

delays affects in a predictable way both the individual output pulse energies and their relative magnitude. An example of

the measured development of both pulse energies vs. the first pulse delay (with fixed delay between the two pulses) is

illustrated in the following graph of Fig. 3b.

Figure 3 : a) Schematic timing arrangement of the flashlamp pump pulse and the dual Q-switched laser output pulses, b) An

example of double pulse laser energy evolutions with delays.

The observed dependence of the plot data can be clearly understood and modelled if the asymmetrical bell-shaped pump-

pulse evolution is considered, along with the fact that the first pulse consumes all the energy accumulated by the pump in

the laser crystal up to the moment of the first Q-switching. This dual-pulse laser mode was tested in LIBS application and

results obtained will be shown in detail later below. Here it should be pointed out however, that our double pulse

configuration is rather different from what is usually employed in typical LIBS practice. In fact, as the two pulses in our

case are separated by several µs, typically from 30 µs to 70 µs, the second pulse never comes in direct interaction with the

plasma created by the first pulse, as the first plasma ionized plume has completely decayed at this later time.



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2.2 LIBS Set Up

A typical optical setup has been used for the LIBS measurements. Two dielectric mirrors are used to guide the laser beam

towards a 50 mm focal length lens that focuses it on the sample surface causing the formation of a plasma plume. The

sample is placed on a 3 axis micrometer stage that allows easy positioning of the point to analyze with respect to the laser

beam focus. The plasma emission is collected by an optical fiber (core diameter of 600 µm and a numerical aperture of

0.22), oriented at 45 ° with respect to the beam axis and at a distance of 15 mm from the sample surface, and transmitted

to the entrance slit of the spectrometer (Fig. 4). Two spectrometer arrangements have been used in the experiments:

- a standard one based on an imaging spectrograph (Jobin Yvon) coupled to an intensified CCD detector (Andor

Technology) gated by means of a pulse generator which allows to resolve the plasma emission.

- a compact one based on small-size monochromator coupled to a CCD array detector from Ocean Optics (HR2000+). By

using a precise pre-trigger pulse from the laser, it is possible to initiate recording of the emission at a relatively short time

interval following the laser pulse, for a minimum integration window of 1 ms. This permits rejection of the strong

continuum emission present at the very early stages of plasma emission. Typical τd values used with the compact

spectrometer were in the range of 1-2 µs.

Figure 4 : LIBS setup

Microscope images of crater created by a laser pulse on brass have been made; its diameter is around 500 µm. As already

mentioned, the energy of the laser pulse can be adjusted by the control box of the system by changing the voltage of the

flashlamp capacitor and/or the Q-switch delay. In this way though, the laser pulse duration might also change. Therefore,

for the variable energy studies, the energy was controlled by placing a variable optical attenuator on the optical laser path.

3 Results and Discussion

3.1 ICCD spectrometer

The laser was tested with regard to its performance in a LIBS experiment by using the ICCD detection system. It provided

the necessary pulse energy density on the sample surface giving rise to material ablation and formation of plasma. Time-

gated spectra, collected from a wide variety of materials, enabled qualitative determination of the elemental composition

in a straightforward way. Time-resolving the spectrum clearly improves the spectra by excluding the broadband



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featureless continuum emission present at early times following laser ablation (Fig. 5 I). In the same manner, a

compromise about output laser energy has to be found in order to obtain the best spectra (Fig. 5 II).

Figure 5: I) Spectra from different time delays a) τd= 120 ns, b) τd= 520 ns, c) τd= 1120 ns ; II) LIB spectra acquired with

different output laser energies

In addition the double-pulse output of the laser was tested and in most cases it led to higher intensity spectra exhibiting

considerably improved signal-to-noise ratio (SNR), as we can see on Fig. 6.

Figure 6: a) LIB spectra and b) Line intensities acquired after single-pulse and double-pulse ablation.

The observed enhancement is expressed as the ratio IDP(E+E)/ISP(2E), where IDP(E+E) is the intensity of a spectral line in the

double-pulse case (two pulses, each of energy E) and ISP(2E) is the intensity of the same spectral line in the single pulse



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case corresponding to the same total energy 2E. The overall emission intensity and the quality of the spectra depended on

the total energy of the pair as well as on the partitioning of the energy between the two pulses and the interpulse delay, ∆t.

To compare the double-pulse against the single-pulse case we investigated the effects of pulse energy on the observed

LIBS signal intensity using brass (Cu-Zn) as a test sample. For simplicity we have chosen to examine the double-pulse

(E+E) with respect to the single-pulse (2E) mode, namely a case in which the total energy is equally divided among the

two pulses in the double-pulse case. In the following graphs (Fig.7 I and II) data corresponding to copper (515.324 nm)

and zinc (472.216 nm) emission lines are used to present the comparison between double and single-pulse modes.

The experimental conditions selected were τd = 1 µs and τg = 5 µs. In the double-pulse mode, the delay between the pulses

was ∆t = 42 µs. For each energy value, the last ten spectra from a total of fifteen pulses were recorded from the same spot

of the sample (first five pulses served to clean the spot and therefore stabilize the intensity). The enhancement of the

signal intensity in the double-pulse case is obvious and can be up to an order of magnitude higher compared to the single

pulse case (Fig. 7 III).

Figure 7 : I) Intensity of a copper emission line at 515.324 nm depending on the laser pulse total energy; a) closed squares

represent the double-pulse (E+E), b) open squares represent the single-pulse configuration (2E) ; II) Intensity of a zinc emission

line at 472.216 nm depending on the laser pulse total energy; III) Enhancement of the signal intensity of selected atomic

emission lines in relation to the total energy emitted on the sample.

It also shows a signal increase with pulse energy indicating that “breaking” a pulse into a pair of two pulses leads to signal

enhancement. However, it is worthwhile to note that the degree of enhancement is different not only between the different

elements (Cu and Zn) but also in atomic emission lines of the same element (Cu). For example, in the maximum energy

applied (26.6 mJ) for Zn the enhancement is 3.8 times, while for Cu it is 10.4 times. This fact can be attributed to the

different energy levels of the spectral lines. Therefore, the 510.554 nm emission line of copper coming from an energy

level equal to 3.82 eV is not so strongly enhanced as the energy increases comparing to the other copper lines that

originate from higher energy levels (6-8 eV). Furthermore, we observe that for 2E<16 mJ no enhancement is observed.

This is explained by the fact that, at this condition, the single pulse energy (E=8 mJ) is very near the ablation threshold

and the plume effects cannot have a strong interaction with the second pulse that follows at a rather late time, namely at ∆t

= 42 µs.



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In order to investigate the possible differences in the plasma characteristics between double and single-pulse, the evolution

of the plasma in time was studied and a comparison between the plasma lifetime of single and double-pulse was

performed. Therefore, three cases of experimental conditions were studied; a) single laser pulse with energy E = 14 mJ, b)

single laser pulse with energy 2E = 28mJ and c) double laser pulse with energy of E+E = 14mJ+14mJ and ∆t = 42 µs. The

integration window was narrow in order to allow adequate time resolution of the emission decay (τg = 100 ns). For each

delay settting, the last five spectra from a total of ten pulses were collected from the same spot of the sample. Then, the

area under selected spectral lines was measured and the average from the five spectra was calculated. In Fig. 8, we follow

the copper and zinc emission lines at 465.112 nm and at 472.216 nm respectively and their intensity over time the plasma

decay for all the different cases.

Figure 8 : The intensity of a zinc emission line when energy of a) E (single pulse), b) 2E (single pulse), c) E+E (double pulse), is

delivered on the sample. For reasons of better depiction, the intensities from single pulse (E) have been multiplied by 1.5

It is evident that the plasma generated by the second pulse lasts longer than the single pulse one. The decay curves yield

an effective time constant for the plasma decay. The single pulse plasma decay constant is measured around 300-400 ns

while the measurement corresponding to the double-pulse condition is around 900 ns.

3.2 Compact spectrometer

Following the initial experiments we continued the tests using a compact, low cost and non-intensified spectrometer

(Ocean Optics HR2000+). The spectrometer is directly triggered by the laser (from control unit) by delivering a trigger

pulse to the spectrometer board well before (7-8µs) the laser pulse. This way acquisition starts typically 1 µs after the laser

pulse with an integration window that is around 1 ms long. Typical spectra obtained with the compact spectrometer are

shown in Fig. 8 a and b. The spectra exhibit, satisfactory spectral resolution over a broad band, allowing the identification

of a wide variety of materials, like pigments in paintings and icons, metals, ceramics, etc…. In Fig 9 a) we show a

spectrum from a red pigment on an icon identified as cinnabar (HgS), since mercury (Hg) was the main detected element.

The existence of calcium (Ca) was due to the underlying preparation layer. In Fig. 9 b) a copper alloy containing tin (Sn=

3.38 %), zinc (Zn= 3.90 %) and lead (Pb= 1.98 %) was also successfully identified.



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The single and double pulse configuration was also tested and compared with this spectrometer. In Fig. 10, I two Brass

spectra obtained with single and double-pulse mode are compared. The Fig. 10 II presents the ratio between two Cu lines

in a Brass spectrum over 50 pulses. This kind of graph allows to compare the stability and repeatability of spectra in single

or double pulse.

Figure 10: I) Spectra obtained from a brass sample a) single pulse (dash line) and b) double pulse (solid line) with the same total

energy ; II) Brass Line Intensities stability graph

The total energy in both cases is 12 mJ, while in the double-pulse configuration the interpulse delay is 20 µs. It is again

evident that in the double-pulse case the spectrum is superior in terms of signal-to-noise ratio and stability.

4 Conclusion

The new laser source exhibits high performance in terms of laser pulse characteristics (output energy, FWHM…) and

repeatability, autonomy and interface. Moreover, its application in the LIBS experiments produces reliable data by

providing the proper laser densities needed to create plasma. Its ability to work also in the double pulse configuration

provides versatility in the measurements leading to increased LIBS signal intensities and improved the SNR. Finally, its

successful interfacing with gated as well as non-gated or non-intensified detectors demonstrates that this new laser is



suitable for integration in compact, portable LIBS instrumentation for the analysis of a wide range of materials. Futures

evolutions of the laser are possible integrating for example an extra tripling frequency stage and a better beam quality.

References

[1] Cremers D A, Radziemski L J (2006) Handbook of Laser-Induced Breakdown Spectroscopy, Wiley, New York, USA

[2] Miziolek A W, Palleschi V, Schechter I, Eds. (2006) “Laser Induced Breakdown Spectroscopy (LIBS): Fundamentals

and Applications”, Cambridge University Press, Cambridge, UK

[3] Palanco S, Laserna J J (2004) “Design considerations, development and performance of a remote sensing instrument

based on open-path atomic emission spectrometry” Review Sci Inst 75:2068-2074

[4] Gronlund R, Lundqvist M, Svanberg S (2005) “Remote imaging laser-induced breakdown spectroscopy and remote

cultural heritage ablative cleaning” Optics Letters 30:2882-2884

[5] Tzortzakis S, Gray D, Anglos D (2006) "Femtosecond laser filaments enable remote LIBS analysis with potential

applications in the monitoring of sculpture and monuments" Optics Letters 31:1139-1141

[6] Anglos D (2001) “Laser-Induced Breakdown Spectroscopy in Art and Archaeology” Appl Spectrosc 55:186A-205A

[7] Corsi M, Cristoforetti G, Giuffrida M, Hidalgo M, Legnaioli S, Masotti L, Palleschi V, Salvetti A, Tognoni E,

Vallebona C, Zanini A (2005) “Archaeometric analysis of ancient copper artefacts by laser-induced breakdown

spectroscopy technique” Microchim Acta 152:105-111

[8] Karen Y.Yamamoto, David A.Cremens, Monty J.Ferris, Leeann E.Foster, Detection of Metals in the environment

using a portable laser-induced breakdown spectroscopy instrument, Appl. Spectrosc. 50 , 1996

[9] Stratis D N, Eland K E, Angel S M (2000) “Dual-pulse LIBS using a pre-ablation spark for enhanced ablation and

emission” Appl. Spec. 54:1270-1274

[10] Noll R, Sattmann R, Sturm V, Winkelmann S (2004) “Space- and time-resolved dynamics of plasmas generated by

laser double pulses interacting with metallic samples” J Anal Atom Spectrometry 19:419-428



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