What really happens inside your cable when the ocean starts pushing back.

When you work subsea, you depend on data you can trust — even when pressure, motion, and temperature shifts are doing everything they can to distort it. Fiber optics help make that possible. They don’t just move information faster; they reveal how your system behaves beneath the surface, in places no human can reach.

This article is the first in a two-part series exploring how light travels through a subsea cable, how fibers respond under mechanical stress, and how sensing technology is reshaping the way engineers monitor offshore systems. Whether you’re designing, deploying, or maintaining complex subsea equipment, understanding these fundamentals gives you a clearer path toward safer, smarter, and more reliable operations.

Let’s start with how fiber optic transmission works — and why it’s becoming the backbone of modern subsea engineering.

A clear look at fiber optic transmission

When you work subsea, you need data you can trust. Even when pressure, movement, and temperature shifts are trying to interfere.

Optical fibers help make that possible. They transmit information at the speed of light and carry far more data than copper ever could, all while remaining resistant to heat, moisture, and chemicals when protected inside stainless steel tubes.

Here’s the short version: Your digital signals (zeros and ones) are converted into pulses of light by a transmitter. These pulses travel through the fiber, reach a detector on the other side, and are translated back into data by a receiver. Fast, clean, and incredibly efficient.

What optical fibers are made of

Inside every subsea cable is a piece of precision engineering.
Optical fibers are drawn from ultra-pure silica and doped with tiny amounts of materials like germanium. A microscopic core carries the light, while a cladding layer keeps it from escaping. Essentially creating a perfectly controlled pathway for the signal.

• Single-mode fibers use a tiny 9-micron core to let one clean mode of light travel long distances.
• Multimode fibers have a wider core that carries multiple light paths — ideal for shorter reaches with high bandwidth.

Because the fibers are so thin, a multicore cable stays compact, even when it houses a high number of fibers. And because power demands are extremely low, fiber optics becomes especially valuable in systems where sensors generate more data than copper could reliably carry.

Fiber optic sensing: when the fiber becomes the sensor

Distributed sensing: when every meter matters

Sometimes you need the entire fiber to act as a sensor — not just parts of it.

This is where distributed sensing comes in. Light is injected into the fiber and reflects back toward the source. Along the way it encounters Rayleigh, Raman, or Brillouin backscattering. These changes reveal what the cable is experiencing in the field:

DAS — Distributed Acoustic Sensing
Real-time detection of vibrations and acoustic signatures (Rayleigh).

DTS — Distributed Thermal Sensing
Long-range temperature monitoring (Brillouin or Raman).

DSS — Distributed Strain Sensing
Tracking deformation and tension along the cable (Brillouin).

DVS — Distributed Vibration Sensing
High-resolution measurement of short-range vibration (Rayleigh).

Ready for part two?

We’ve only touched on what fiber optic sensing can do. In the next article, we explore real-world applications, from protecting offshore energy assets to supporting defense missions and advancing marine research.

Curious how fiber optics could support your next subsea project? Book a meeting with our colleague George Brandenburg. We’re always ready to co-create the next solution.