GB2180059A

GB2180059A - Plasma spectroscopy

Info

Publication number: GB2180059A
Application number: GB08522104A
Authority: GB
Inventors: Kevin Christopher Byron
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1985-09-05
Filing date: 1985-09-05
Publication date: 1987-03-18
Also published as: GB8522104D0

Abstract

A remote plasma spectrometer comprises a composite optical fibre (2) via which high power pulses from a laser (1) are delivered to an article under investigation (4) and which serves also to detect photons in the plasma produced at the article and to deliver them to photon counting equipment (5) at a wavelength determined by a monochromator (6). By counting at different wavelengths a "finger print" of the article can be built up. The spectrometer is calibrated by use of known substances in place of the article (4). Fibre (2) may comprise a core (11-Figure 2) for the transmission of the laser pulses to the article, and a surrounding layer (13) for the transmission of photons from the article, a cladding layer (14). The refractive index of the core (11) is greater than that of the surrounding layer (13), which is greater than that of the cladding (14). <IMAGE>

Description

SPECIFICATION Plasma spectroscopy This invention relates to plasma spectroscopy and in particularto remote plasma spectroscopy.

In conventional plasma spectroscopy a high power laser pulse is caused to be incident on a material under investigation. A plasma is produced at the surface ofthe material and study of characteristic emission lines in the plasma allow determination of the material's constituents.

According to one aspect ofthe present invention there is provided a plasma spectrometercomprising a source of high power laser pulses, means to deliver the laser pulses to an article or partthereof underin- vestigation, said means serving also to detect photons in the plasma produced at the article by the laser pulses, and means to produce an outputfrom the detected photons indicative ofthe constituents of the article.

According to a further aspect of the present invention there is provided a method of investigating the constituents of an article or a parttherof comprising the steps of delivering high power laser pulses to the article byway of a first means, employing thefirst means to detect photons in the plasma produced at the article by the laser pulses and to conductthe photons to second means whereby an output indicative of the constituents ofthe article is produced.

According to another aspect ofthe present invention there is provided a composite optical fibre, for use in remote plasma spectroscopy applications, comprising a core of a first refractive index along which laser pulses can be transmitted to an article or part thereof under investigation, a cylindrical layer, coaxial with the core and of a second refractive index, along which photons detected in a plasma produced at the article by the laser pulses can be transmitted in the opposite direction to the laser pulses, and a cladding layer of a third refractive index disposed on said cylindrical layer, the first refractive index being grea- terthan the second refractive index, and the second refractive index being greater than the third refractive index.

According to yet another aspect ofthe present invention there is provided a composite optical fibre comprising a core of a first refractive index and small core cross-sectional area, a cylindrical layer coaxial with the core, the cylindrical layer being of a second refractive index with a large numerical aperture, and a cladding layer of a third refractive index disposed on said cylindrical layer, the first refractive index being greater than the second refractive index, and the second refractive index being greater than the third refractive index.

An embodiment of the invention will now be des- cribedwith reference to the accompanying drawings, in which: Figure 1 shows schematically a remote plasma spectrometer including a composite optical fibre; and Figure2 shows a section through the composite optical fibre and the associated optical paths thereof.

The output of a high power laser 1 (Figure 1 )for example an Nd,YAG or ruby laser is applied to one end of an optical fibre 2 via a beam splitter3. The other end ofthe optical fibre is located close, for ex- ample 1mm,toan article 4 under investigation. The composite optical fibre 2 is such as to permit a laser pulse from laser 1 to be delivered to article 4 and then to simultaneously detect the spectroscopic signal in the plasma produced at the surface ofthe article and transmit it to a photon counting system 5 via the beam splitter 3. A monochromator 6 serves to tune the detected spectroscopicsignal to a particular wavelength so that the system 5 can count the photons at that particular wavelength.By counting photons at various different wavelengths a "finger print" of low level impurities etc in the article 4 can be obtained. The spectroscope is calibrated by use of known substances in the place of article 4.

The plasma is produced without the necessity for lenses merely by bringing the fibre end close to the material under test. The laser and photon counting equipment can be disposed at a position remote from the material under test due to the use of a fibre to deliverthe laser pulse to the material and to detect the photon output, which fibre effectively comprises a probe which may be positioned in relatively inac cessible places to test material thereat. Typical app- lications of plasma spectroscopy include corrosion detection in reactors, tanks etc, archeological invest- igations and forensic testing. The remote plasma spectrometer employs an essentially nondestructive technique since only a small pit of a few microns depth and width is produced.

The basic requirements of the single composite fibre 2 are the ability to transmit laser light with a high power density and the provision of a suitabie collection area for the returning spectroscopic signal. A suitable coaxial fibre design comprises a core with a small cross-sectional area for transmitting the laser light and a surrounding cylindrical layerwith a large numerical aperture (N.A.) and providing a large collection area. Such an arrangement is shown in Figure 2 and comprises a core 11 forthe transmissionoflaserlighttoa pointl2,forthegener- ation thereat of a plasma, a cylindrical layer 13 forthe return path for photons detected in the plasma at point 12 and a cladding layer 14.The refractive index n1 of the core 11 is greater than the refractive index n2 of the layer 13 which is greaterthan the refractive indexn3ofthecladding layerl4(n1 > n2 > n3).

The core 11 may be of highly doped GeO2 or pure GeO2, the layer 13 may be of moderately doped GeO2, and the cladding layer 14 may be of pure SiO2.

Further protective layers may be applied to layer 14 as required. Typicallythe radius of core 11 is ofthe order of 5 to 50 > m whereas the outer radius of layer 13 is of the order of 50 to 200gum. In the case of plasma spectroscopy applicationsthetransmission loss forthe core 11 is unimportant since high powers over relatively short fibre lengths are concerned.

However the return path, layer 13, should be as low in attenuation as possible in order to obtain the maximum transmitted signal from the plasma at point 12.

Claims

1. A plasma spectrometer comprising a source of high power laser pulses, means to deliverthe laser pulses to an article or partthereof under investigation, said means serving also to detect photons in the plasma produced at the article by the laser pulses, and means to produce an output from the detected photons indicative ofthe constituents ofthe article.

2. A plasma spectrometer as clai med in claim 1 wherein the meansto deliverthelaserpulsesandto detectthe photons comprises a composite optical fibre.

3. A plasma spectrometer as claimed in claim 1 or claim 2 wherein said output producing means comprises a photon counting arrangement and a mono chromatorwherebyto enable counting of photons at predetermined wavelengths.

4. A plasma spectrometer substantially as herein described with reference to and as illustrated in Figure 1 with or without reference to Figure 2 ofthe accompanying drawings.

5. A method of investigating the constituents of an article or a partthereof comprising the steps of delivering high power laser pulses to the article by way of a first means, employing the first means to detect photons in the plasma produced atthe article bythe laser pulses and to conduct the photons to second means whereby an output indicative ofthe constituents of the article is produced.

6. A method as claimed in claim 5 wherein the first means is a composite optical fibre.

7. A method as claimed in claim 5 or claim 6 wherein the second means comprises a photon counting arrangementand a monochrometerand including the steps of setting the monochromatorto a respective particular wavelengths and counting the numbers of photons thereat.

8. A method of investigating the constituents of an article or a partthereofsubstantially as herein described with reference to Figure 1 with or without referenceto Figure 2 of the accompanying drawings

9. A composite optical fibre, for use in remote plasma spectroscopy applications, comprising a core of a first refractive index along which laser pulses can be transmitted to an article or partthereof under investigation, a cylindrical layer, coaxial with the core and of a second refractive index, along which photons detected in a plasma produced at the article bythe laser pulses can be transmitted in the opposite direction to the laser pulses and a cladding layer of a third refractive index disposed on said cylindrical layer, the first refractive index being grea terthan the second refractive index, and the second refractive index being greater than the third refractive index.

10. Acompositeopticalfibre as claimed in claim 9, wherein the core is of highly doped pure GeO2, the cylindrical layer is of moderately doped GeO2 and the cladding is of pure SiO2.

11. A composite optical fibre substantially as herein described with reference to Figure 2 of the accompanying drawings.

12. A composite optical fibre comprising a core of a first refractive index and small core crosssectional area, a cylindrical layer coaxial with the core, the cylindrical layer being of a second refractive index with a large numerical aperture, and a cladding layer of a third refractive index disposed on said cylindrical layer, the first refractive index being grea- terthan the second refractive index, and the second refractive index being greater than the third refractive index.