Sensing fiber optic cables can be used not only for data transmission, but also to measure temperature, strain rate and acoustic signals in harsh environments. AP Sensing’s Distributed Temperature Sensor (DTS) and Distributed Fiber Optic Sonic/Vibration Sensing (DAS & DVS) solutions enable efficient monitoring of critical infrastructure and buildings such as railway and road tunnels, pipelines, bus ducts and parking lots .
Optical cables generally have three components: core, coating, and cladding. The cladding core has a different index of refraction, which is the speed at which light travels through the material. By minimizing the normal critical angle, maximum total internal reflection is achieved, so light can travel for kilometers with minimal attenuation.
Our sensor cables (sensing fiber optic cables) are completely passive and available in a variety of different assemblies and configurations, including metal tube, metal-free, sleeved or armored stainless steel. Metal-free fiber optic cables reduce the risk of induced voltages and are generally more flexible, while metal-armored cables are more robust and effectively prevent animal damage, making them the right choice for harsh environments. In addition, a variety of suitable jackets are available, such as flame retardant non-corrosion (FRNC) jackets, watertight high density polyethylene (HDPE) or other types.
To strengthen and protect the core and cladding, a coating layer is also applied. The appropriate coating for the fiber can be selected based on the applicable temperature range and sensing technology. Sensing fibers for standard temperature ranges use acrylate coatings, while fibers for higher temperature ranges or low temperature environments use polyimide coatings.

Precoat 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

Sensing cables are available in multimode (MM) and singlemode (SM) fibers or a combination of both. For MM fiber, a core diameter of 50µm or 62.5µm is usually chosen, which transmits significantly more light in the core than SM fiber. Currently, a 50µm core is more desirable than a 62.5µm core in most cases and has become the established standard for multimode fiber. Most MM fibers also have a graded index (GI) in cross section. That is to say, the index of refraction between the cladding and the core is gradually transformed, and for stepped index fibers, the index of refraction from the core to the cladding decreases sharply (mainly used for single-mode fibers) .

When the initial pulse of laser light is launched into the fiber, the dominant mode of light propagation is along the central fiber axis. If light enters the fiber at an angle to the centerline, internal reflections occur, causing the light to travel in a step-like (zig-zag or helical path) in the fiber. Some rays have a longer optical path than others and will arrive after the dominant mode light. The graded index of MM fiber allows non-dominant mode light to propagate faster due to the lower index of refraction near the edge of the fiber. This minimizes the effects of modal dispersion.

Typically, MM fiber is used for DTS and most high bandwidth fiber optic communication links. MM fibers have a larger cross-section and therefore are able to generate a larger amount of light that can be coupled into the core and reflected within the core. Compared to SM cores, MM cores minimize core alignment mismatches, so less light is lost at splices and mechanical connections. This enables DTS systems to have better signal-to-noise ratio (SNR) and resolution performance.

However, SM fiber is suitable for different systems. SM fibers typically have a small core with a diameter of 9µm. It only allows light to travel in one mode, thus minimizing modal dispersion. Less light is coupled in the fiber, so it is more challenging to obtain measurement data from the Raman scattered signal. Since the Rayleigh scattering signal is several orders of magnitude stronger than the Raman scattering signal, SM fibers are mostly used in DAS or DVS systems, which use the Rayleigh scattering signal.

Sensing fibers can be used even in remote locations because these cables are small and require no electricity to operate. Sensing cables are neither electromagnetic interference (EMI) nor conductive, so they can be used where high voltage electricity or flammable materials are present, such as jet fuel in airport hangars.