Linear Heat Detection Cables (Fiber Optic)

Fiber-optic cables typically have three components: the core, the coating and the cladding. The cladding and the core have different refractive indices, which is the speed of spreading light in a material. By minimizing the normal critical angle, the maximum of the total internal reflection is achieved, so light can travel for kilometers with minimal attenuation.

Our sensor cables are completely passive and available in a variety of different compositions and configurations including metal-tubing, metal-free, tube-in-tube or armored stainless steel. Metal-free cable reduces the risk of induced voltages and are usually flexible, while metal armored cables have high rodent protection, are robust and the right choice for harsh environment. Additionally, a wide range of appropriate sheathings are available, for example flame retardant non-corrosive (FRNC) sheathings, water-tight high-dense polyethylene (HDPE) or others.

For strengthening and protection of the core and cladding, a primary coating is applied. The coating of fibers needs to be selected for the applicable temperature range and sensing technology. Sensing fibers for standard temperature ranges utilize acrylate coating, while fibers for higher temperature ranges or cryogenic environment use polyimide coating.

Primary coating ambient temperature range

• Acrylate -40 °C to +90 °C
•  High Temperature Acrylate  -40 °C to +150 °C
•  Silicone PFA  -40 °C to +200 °C
•  Polyimide -180 °C to +300 °C
•  Metal >300 °C

Sensor cables are available with multimode (MM) and singlemode (SM) fibers or a combination of both. For MM fibers, typically a core of 50 µm or 62.5 µm diameter is chosen, which enables significantly more light to travel in the core than in SM fibers. Nowadays, a 50 µm core is preferred over 62.5 µm in most cases and has become the established standard for multimode fibers. Most MM fibers also have a graded index (GI) of their cross-section. This means that the transition in index of refraction is gradual between the cladding and the core, as opposed to step-index fibers, where the refractive index sharply decreases from the core to the cladding (mostly used for singlemode fibers).

The main mode of light propagation is along the central fiber axis, when an initial pulse of laser light is emitted into an optical fiber. If light enters the fiber at an angle to its centerline, this results in internal reflections causing the light to travel in the step-index (zigzag or spiral path) through the fiber. The optical path of some light rays will be longer than others and will arrive after the main mode light. Due to the lower index of refraction near the edge of the fiber, the GI of MM fibers allows non-main modes of light to travel faster. This minimizes the effect of modal dispersion.

Typically, MM fibers are used for DTS and for most high-bandwidth fiber optic communication links. MM fibers have substantially greater cross-sections that enable a higher volume of light, which can be coupled into and then internally reflected in the core. Compared to the core of a SM fiber, MM fiber minimizes the core alignment mismatch, so less light gets lost at splices and mechanical connectors. This results in a better signal-to-noise ratio (SNR) and resolution performance for DTS systems.

However, SM fibers are advantageous for different systems. The SM fibers typically have a small core of 9 µm in diameter. By allowing light to propagate in only one mode, modal dispersion is minimized. Less light can be coupled into the fiber, so obtaining measurements from the Raman scattering signal is much more challenging. As the Rayleigh scattering signal is orders of magnitude more intense than the Raman scattering signal, SM fibers are most used for DAS or DVS systems, as these systems utilizes the Rayleigh scattering signal.

Fiber optic sensor cables are usable even at remote locations, as these cables are very small in size and need no electrical power to function. The sensor cables are also immune to electromagnetic interference (EMI) and do not conduct electricity, so they can be used in locations with high voltage electricity or the presence of flammable material such as jet fuel in airport hangars.