Department of Pure and Applied Chemistry/CPACT

Notes prepared by

Professor David Littlejohn

Department of Pure and Applied Chemistry/CPACT

University of Strathclyde

Cathedral Street Glasgow

G1 1XL

UK

Revised October 2007

5. Optical Fibres

5.1 Transmission of light through a fibre

Optical fibres are invaluable in on-line, in-line and non-invasive spectroscopic analysis as they enable the removal of the analyser from hazardous areas, can facilitate the acquisition of data from otherwise inaccessible locations and remove the need for extractive sampling, as well as giving the option of multiplexing.

An optic fibre (Figure 17) consists of a cylindrical core of refractive index (n1) that is higher than the refractive index of the annular outer cladding (n2). Such an optical fibre is referred to as a step-index optical fibre as both n1 and n2 are uniform in the core and the cladding regions. For example, a fibre for NIR consists of a high purity silica core with a doped silica cladding [1].

Figure 17 – a cross section through an optical fibre

In considering the propagation of light along the fibre, the angle of refraction, θr, of the meridianal ray can be related to the angle of incidence, θi, at the air / core interface by:

sin θi = n1 sin θr

The angle of incidence θ at which the refracted ray strikes the core-cladding interface is thus (90 - θr). The ray must be totally reflected at the core-cladding interface for the ray to continue to propagate along the fibre. In order for total internal reflection to occur, the minimum value of θ is given by:

n1 sin θmin = n2

Total internal reflection in a fibre occurs at θi if sin θi ≥ n2/n1

As sin θi = cos θr and cos2θr = 1 – sin2θr

≥ 1 – sin 2θr

n22 ≥ n12 - n12 sin 2θr

n22 - n12 ≥ - n12 sin 2θr

(n12 n22) ½ ≥ n1 sin θr

Snell’s Law : no sin θi = n1 sin θr (where no is the refractive index of air)

Hence for maximum θi at which total internal reflection occurs (θi, max) substitution for the right side of the above expression gives:

no sin θi, max = (n12 - n22)½

This can be further simplified by assuming no = 1

5.2 Fibre Types

Different types of optical fibre are chosen for their suitability for a particular application. Different core materials have different transmission performances and are chosen to give optimum transmission at the wavelengths required for a given technique. Below (Table 4) are a few examples collated by John Andrews of Clairet Scientific [1]:

Table 4 – Relative properties of fibre optic materials.

Wavelength Range / Fibre Material / Maximum Length / Notes
Mid-IR
500-5000 cm-1 / As2S3
Chalcogenide
Silver Halide / 6 m
15 m / Expensive; low frequency cut off at 1000 cm-1, absorbs at 3300 and 2500 cm-1
Visible light sensitive; however, robustness improved greatly recently
Extended Near –IR
2250-6000 cm-1 / Fluoride
Glasses / 50 m / Good transmission characteristics
Near-IR
5000-10000 cm-1 / low OH Silica / > 1000 m / Low cost with excellent transmission. Silica must be “dry” to avoid strong OH absorption
Visible and UV
10000-50000 cm-1 / Silica / 200 m / High energy UV can cause radiation damage in fibre

There is little choice of fibre materials for Mid-IR. An alternative to the optic fibre exists in the form of Axiot modules. These consist of lengths of steel tubing, with either gold or a nickel internal coating, connected with plane mirrors at 90 to one another. Parallel light introduced to these makes grazing angle reflections and remains essentially collimated. Right angle joints at the ends of the tubes can rotate to allow this light guide to be ‘plumbed in’ and gold inner coating can give a maximum run of 25 meters with reasonable transmission efficiency.

5.3  Problems with Optical Fibres

All fibres suffer from losses as light is attenuated by absorbance by the core material and imperfections of the total internal reflection [3]. Moving fibres during the transmission of information can also cause problems as changes in the angle of incidence can effect the total internal reflection of the fibre and hence the final spectrum.

The diameter of fibres has consequences: thin fibres have greater flexibility, cost less and have less attenuation whilst larger diameter fibres carry more light and are easier to interface to spectrometers. Alternatively, bundles of thin fibres can be used, incorporating the best properties of both. However, this can be cost prohibitive with exotic materials like chalcogenide.

The coupling of fibres to the light source and analyser can affect the efficiency of the fibre. Overfilling occurs when the diameter of the radiation beam is greater than that of the fibre core. Much of the radiation is lost to the fibre cladding. Underfilling is the result of the diameter of the radiation beam being much less than that of the core.Here the core is not being used to its full light carrying capacity. The light source must also be matched to the numerical aperture of the fibre. Fibre couplers clamp the fibre and allow movement in all three axes as well as pitch and yaw movement. Movable lenses guide the source radiation into the fibre.

5.4 Probe Designs

There are several probe designs available for use with optical fibres [1]. Reference will be made to the different probe types in the following sections, but a general introduction is given here:

Figure 18 A Transmission Probe 1 Excitation fibre. 2 Lens. 3 Light guides. 4 Retroreflector. 5 Sample gap. 6 Lensed window. 7 Scattered light fibre.

Transmission probe (figure 18) – Light from the source is taken to the end of the probe and bent through 180o before passing through a small sample filled gap and then into the return fibre. These are best for quantitative measurements.

Figure 19 – A transflection Probe. 1 Collecting fibre. 2 Illuminating

fibre. 3 Sample area. 4 Mirror.

Transflection probe (figure 19) - here the light exits the input fibre, passes through the sample before being mirror-reflected back into the return fibre. These are easier to manufacture and therefore cheaper than transmission probes.

Both of the above probes are used for in-line or in situ analysis. The sample liquid fills the sample gap or sample area to allow a spectrum to be obtained.

Figure 20 – Attenuated Total Reflection Probe. 1 Collection fibre. 2

Illuminating fibre. 3 ATR crystal.

Attenuated Total Reflection (figure 20) - this involves sending the light through a transmitting crystal at the end of a probe where it picks up spectroscopic information at the sample – crystal interface. Hence, this probe is also used in-line and in-situ. The fibre end itself can act as the crystal or materials like diamond can be used to cope with hostile conditions. ATR crystals have a very short equivalent path length, as the penetration depth of the evanescent wave into the sample material is small at each reflection (bounce). This means that ATR probes are very useful for analyzing strongly absorbing compounds. Various designs of ATR probe can be used to give different absorption pathlengths through different numbers of reflections at the interface between the crystal and the sample.

Probes used in Raman scattering spectrometry will be described in a later section.

5.5 Multiplexing with Optical Fibres

If the source of radiation is intense enough, it is possible to split the light and take it to different sampling points using a multiplexer. As the figures below indicate [3], the incident radiation can be multiplexed in time, using a moveable translation stage (figure 21) or in space using a star coupler (figure 22), which allows light to pass simultaneously through several optical fibres. This latter approach is only feasible if the light source is intense (e.g. a laser for Raman spectroscopy) and there is little attenuation of the radiation through the fibres. Normally a second system of multiplexing is used to collect transmitted or reflected/scattered radiation and take it to a detector or series of

detectors for analysis.

Figure 21 Time multiplexing using a moving translation stage.

1 Source. 2 Output to sampling points.

Figure 22 Space multiplexing using a “star coupler”. 1 Source. 2 Output to sampling points.

5.6 Review Questions

1. Why is mid-IR spectrometry the least easy optical technique to use

with remote probes?

2. What are the main differences between transmission, transflectance

and ATR probes? Which of these probes is least useful for

analysis of liquids with suspended particles and why?

3. When would multiplexing be useful in process analysis with

spectroscopic techniques? What are the possible disadvantages of multiplexing?

5.7 References

1. A Guide to Remote Spectroscopy Using Fibres and Light Guides

Technical Briefing from Clairet Scientific Ltd., Northampton, 1999.

2. Raman Spectroscopy With Fibre Optics

I. R. Lewis and P. R. Griffiths

Applied Spectroscopy, 1996, 50, 12A – 30A.

3. Raman Spectroscopy for On-line, Real-time, Multi-point Industrial

chemical analysis.

M.J. Roberts, A.A. Garrison, S.W. Kercel and E.C. Muly

Process Control and Quality, 1991, 1, 281-291.

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