Use of fibre optics in NIR online spectroscopy
author: cily
2023-01-03
One important area for NIR online spectroscopy is the use of fibre-optic cables to carry the light from the spectrometer to the NIR process cell. It is often overlooked as a crucial item in the set-up of a successful NIR online system, from a performance , but also from a cost point of view.
Our lab-grade and premium-grade UV-Visible optical fiber assemblies are durable, high-quality patch cords that deliver uniform results with minimal signal variance. Patch cords act as both illumination and read fibers and connect easily to Optosky spectrometers, light sources and sampling accessories.
Types of fibre-optic cables
Fibre optics distances covered in the NIR spectroscopic region can be several hundred metres long, as they originally have been designed for the telecommunications industry to achieve long distances with very low light loss. Once the light is in the NIR fibre very little is lost due to absorption or light exiting the fibre. There are a lot of different types of fibre optics, with variations in core diameter (200 μm-600 μm), numerical apertures between 0.22 and 0.28 (the higher the more light you can squeeze into them), coating (polyimide and polyacrylate) and armor (stainless steel, polypropylene and other polymers) for protection.
If you run optical fibres for very long distances cost can be an issue, with the 200 μm fibres generally cheaper than the larger core ones. If you choose the smaller diameter with a higher numerical aperture the light levels received by the spectrometer might be sufficient, plus the fibres can also be spliced in the field (if they are coated with polyacrylate). Unfortunately, polyacrylate is not rated for usage at higher temperatures, you would have to use polyimide for that. There are now fibre-optic cables available which can be used way beyond 200 oC, a temperature which might be experienced directly at the process cell connection.
Process NIR fibres could be run in a tray or conduit together with other electrical or signal cables , but should have a very distinguishable colour and labels to highlight them as special.
Junction Boxes
Another useful option could be an additional junction box close to the NIR cell with a short run of fibre. Most likely the optical fibre might get damaged directly at the NIR cell due to high temperatures, mechanical failures etc., so if you have a junction box close by you would only have to replace the last 5 m. This can save you cost and more crucially time. In the junction box you could either use SMA connectors or splicing to couple light from the long distance run into the short fibre. The quality of the SMA connectors is crucial though, sometime Etalon artefacts have known to creep into the absorption spectra via the SMA connection.
Collecting Backgrounds
For collecting background spectra two options can be considered, either through the empty NIR absorption cell only at the start or through a separate reference loop of fibre-optic cable at a suitable length. Both methods have their advantages and disadvantages. If the spectrometer is housed in a controlled temperature environment the first option might be advantageous, whereas in the other case any changes in the throughput of the spectrometer due to temperature or other changes will be captured by taking regular backgrounds.
Our lab-grade and premium-grade UV-Visible optical fiber assemblies are durable, high-quality patch cords that deliver uniform results with minimal signal variance. Patch cords act as both illumination and read fibers and connect easily to Optosky spectrometers, light sources and sampling accessories.
Types of fibre-optic cables
Fibre optics distances covered in the NIR spectroscopic region can be several hundred metres long, as they originally have been designed for the telecommunications industry to achieve long distances with very low light loss. Once the light is in the NIR fibre very little is lost due to absorption or light exiting the fibre. There are a lot of different types of fibre optics, with variations in core diameter (200 μm-600 μm), numerical apertures between 0.22 and 0.28 (the higher the more light you can squeeze into them), coating (polyimide and polyacrylate) and armor (stainless steel, polypropylene and other polymers) for protection.
If you run optical fibres for very long distances cost can be an issue, with the 200 μm fibres generally cheaper than the larger core ones. If you choose the smaller diameter with a higher numerical aperture the light levels received by the spectrometer might be sufficient, plus the fibres can also be spliced in the field (if they are coated with polyacrylate). Unfortunately, polyacrylate is not rated for usage at higher temperatures, you would have to use polyimide for that. There are now fibre-optic cables available which can be used way beyond 200 oC, a temperature which might be experienced directly at the process cell connection.
Process NIR fibres could be run in a tray or conduit together with other electrical or signal cables , but should have a very distinguishable colour and labels to highlight them as special.
Junction Boxes
Another useful option could be an additional junction box close to the NIR cell with a short run of fibre. Most likely the optical fibre might get damaged directly at the NIR cell due to high temperatures, mechanical failures etc., so if you have a junction box close by you would only have to replace the last 5 m. This can save you cost and more crucially time. In the junction box you could either use SMA connectors or splicing to couple light from the long distance run into the short fibre. The quality of the SMA connectors is crucial though, sometime Etalon artefacts have known to creep into the absorption spectra via the SMA connection.
Collecting Backgrounds
For collecting background spectra two options can be considered, either through the empty NIR absorption cell only at the start or through a separate reference loop of fibre-optic cable at a suitable length. Both methods have their advantages and disadvantages. If the spectrometer is housed in a controlled temperature environment the first option might be advantageous, whereas in the other case any changes in the throughput of the spectrometer due to temperature or other changes will be captured by taking regular backgrounds.
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