Welcome to the first article of our two-part series on fiber optic technologies. In this initial exploration, we focus on the fundamentals of fiber optic transmission and basic sensing technologies. These advancements are not only redefining the limits of data communication but also opening new frontiers in environmental and industrial monitoring.

A Brief Intro to Fiber Optic Transmission

Optical fibers can transmit data at the speed of light – much faster than copper wires conduct electricity. These fibers are less susceptible to high temperatures, moisture or harsh chemicals when placed in a metal/stainless steel tube, and can transmit orders of magnitude more data than over a twisted pair. 

Here’s how it works. Digital data comes in the form of a series of zeros and ones. These feed into a transmitter which processes that data into blocks which are sent through the fiber as light pulses from a laser or LED. At the other end of the fiber, a detector collects the pulses and a receiver translates them back to output data.

What are optical fibers?

Optical fibers are made of pure silica doped with tiny amounts of other materials, such as germanium. The silica is drawn into an extremely thin, extremely long glass fiber.

This fiber consists of a core which is the medium that transmits the light pulses. The core is surrounded by a cladding made up of a slightly different glass composition. This acts as a mirror, preventing the light from escaping the core. The core and cladding are protected by a buffer and an outer protective coating. 

In a single mode fiber, the core is a tiny nine micrometers in diameter, or about 1/3 the thickness of a sheet of paper. This fiber core is so tiny that it only allows a single mode of light transmission to pass through. In multimode fiber the core is larger, about 62 microns. The larger diameter allows multiple light pulses to travel through the core at the same time, although the signals can’t travel as far as with single mode fiber. 

Since the fibers are so thin, a cable containing multiple fibers can be made very compact. Additionally, by using optical fibers for data transmission, the power requirements for the cable are very low for transmitting large amounts of information.

This becomes an advantage when it comes to carrying fiber optic sensor data, because the better the sensor, the more data it produces, and the harder it is to transmit that data. 

Fiber Optic Sensing Explained 

In fiber optic sensing, just as in data transmission, the light travels through a glass fiber, but with a difference. Lasers transmit light at a specific frequency and intensity. But when the fiber experiences certain changes, such as strain or an increase in temperature, the frequency or the intensity of the light traveling through it changes and this change can be detected. 

One way to sense changes in a fiber is with a fiber Bragg grating. The grating is a short section of fiber that has been treated to change the refractive index of the core. When the grating undergoes strain or changes in temperature, the corresponding wavelength of light passing through will change. This change directly correlates to the change acting on the fiber and can be detected and analyzed by an interrogator at the end of the fiber. 

So, for example, with a series of fiber Bragg gratings situated every meter along a 150-meter fiber, any changes in strain or temperature could be pinpointed at certain locations.

Distributed Sensing Technology with Fiber Optic Sensing

Another type of sensing uses the entire length of the fiber as a sensor. With this kind of “distributed sensing,” the light from a laser is injected into a single mode fiber. As the light travels down the fiber, bounces off the far end and travels back to the source, it can become scattered, altering the frequency or intensity of the light waves. This backscattering may be caused by bending or movement of the cable, changes in temperature, or cable vibrations.

Rayleigh backscattering, Brillouin backscattering, and Raman backscattering are different kinds of light scattering patterns which can be used to detect and interpret these losses to the light signal. Each one can be used to find out from a distance what has happened to the cable under the sea: 

• DAS, Distributed Acoustic Sensing (Rayleigh backscattering)
• DTS, Distributed Thermal Sensing (Brillouin or Raman backscattering)
• DSS, Distributed Strain Sensing (Brillouin backscattering)
• DVS, Distributed Vibration Sensing (Rayleigh backscattering, short distance)

Learn more about the applications

We've only scratched the surface of what these incredible innovations can achieve. In our next blog post, we will delve into the exciting and diverse applications of distributed fiber optic sensing. From safeguarding critical subsea infrastructure to enhancing environmental research, the potential uses are as vast as the ocean depths themselves.

Join us as we uncover how distributed fiber optic sensing is revolutionizing industries, providing unprecedented insights into the mysteries of the undersea world, and playing a pivotal role in defense and energy sectors. Curious to discover possibilities for your next project?

Book a meeting with our colleague George Brandenburg.