An optical fiber is roughly the thickness of a human hair and can transmit massive amounts of data though light. Fiber optic cables offer superior characteristics to metallic cabling such as weather resistance, faster transmission speeds, and higher bandwidth. First widely used in the telecommunication industry, fiber optics are used in many applications such as networking, dentistry, space, automotive and sensing.
Because the fiber is immune to electrical interference and does not conduct electricity, there are no signal disruptions between cables (cross talk) or from other equipment. This makes them a great choice for high voltage monitoring applications, or in locations prone to lightning strikes.
The two main categories of optical fiber revolve around the number of modes of light propagation: Multi-mode fiber (MMF) and Single-mode fiber (SMF).
MMF has a large core which allows different modes of light propagation, but is limited in transmission length (typically less than 1000 meters) due to the modal dispersion of light along the length of the fiber. Modal dispersion means that different rays of light enter the core at different angles to the axis of the core, resulting in different arrival times of data packets. You can image that shallower angles towards the center axis take more direct path than larger angles. The difference in paths lengths limits the bandwidth of the signal, which is why MMF is commonly used within data centers or offices with short connection lengths.
SMF allows only a single mode of light to propagate along the length of the fiber, reducing modal dispersion enabling links to be 10’s of kilometers in length. SMFs are often used in high-precision scientific research because restricting the light to only one propagation mode allows it to be focused resulting in a less noisy signal. For our optical sensors, this feature is beneficial for remote sensing over long distances such as in Oil & Gas wells or on large structures.
1. Glass Core Light travels through the glass core to transmit data. The core diameter is around 8-11µm in single mode fiber (SMF) and around 50-100µm in multi mode fiber (MMF). Fibos uses SMF.
2. Cladding The cladding has a slightly lower refractive index than the core does. The difference between the refractive indices of the core and cladding keep light contained within the core by creating internal reflection between the two layers. This enables the light to travel continuously through the fiber in a path, rather than scattering out through the sides, resulting in a clear signal. Cladding diameter is 125µm for the typical fiber type Fibos uses.
3. Buffer A buffer material surrounds the cladding and core to protect it against cracks. For standard fiber applications, the buffer layer is usually made of acrylate, a soft polymer. When higher temperature or corrosive environments are expected, the buffer can be polyamide or metallic.
4. Jacket The final layer is a jacket which is provides weathering protection. The cable jacket is the first line of moisture, mechanical, flame and chemical defense for a fiber cable. Depending on the application, they may be a polymer based material or stainless steel. Diameters range from 900µm-3mm.
The type of optical fiber that is used depends on the application. For most of our sensors, Fibos uses Corning ClearCurve SMBI-5-XMT. This fiber has a minimum bend radius of 5mm and an acrylate coating that can survive 180°C continuously.
For higher temperature applications, Fibos will produce the FBG within the ClearCurve fiber and then remove the coating and splice the FBG to another optical fiber cable. Polyimide fiber can survive ~300°C (Nufern), copper coated fiber can survive ~450°C (IVG) and gold coated fiber can survive up to ~700°C (Fiberguide).
The production of optical fiber is now well known but it is an expensive process to develop. There are many trade secrets that come with the production of optical fiber, and consistent quality over long lengths of optical fiber is difficult to achieve. For this reason, Fibos has selected optical fiber from known manufacturers such as Corning and Nufern.