Is the light inside the fiber colorful?

International Zone (English) OFSCN Knowledge Base (Public)
1

Can we see the light traveling through the fiber with the naked eye? Why is it referred to as “invisible light”?

2

In fiber optic communication and FBG (Fiber Bragg Grating) sensing, the light used is generally not visible to the naked eye. Here is the technical explanation for why this is the case:

1. Wavelength and the Electromagnetic Spectrum

Visible light occupies a very narrow band of the electromagnetic spectrum, typically between 380 nm and 750 nm.

In contrast, most fiber optic systems and OFSCN® FBG sensors operate in the Near-Infrared (NIR) region. The most common “windows” for transmission are:

  • 850 nm (Short-haul/Multimode)
  • 1310 nm (C-band start)
  • 1550 nm (The standard for high-performance sensing and long-distance communication)

Because these wavelengths are longer than 750 nm, they fall outside the detection range of the human retina, making them “invisible light.”

2. Why use invisible light?

The choice of the 1550 nm range (the C-band) for OFSCN® products is based on physical efficiency:

  • Lower Attenuation: Silica-based optical fibers have their lowest signal loss (attenuation) near 1550 nm.
  • Material Properties: At these wavelengths, the light experiences less scattering and absorption compared to visible light.

3. Safety Warning

Even though you cannot “see” the light, it is dangerous to look directly into a powered optical fiber.

  • Invisible Thermal Damage: High-power infrared light can still be focused by the eye’s lens onto the retina, causing permanent thermal damage or blindness without the user feeling any immediate pain or seeing a bright flash.
  • OFSCN® Equipment: When working with our Fiber Bragg Grating Interrogators, always ensure the laser source is deactivated before inspecting connectors.

4. Technical Diagnostics

To “see” this invisible light, engineers use specialized tools:

  • Infrared Detection Cards: These convert NIR light into visible glow.
  • Visual Fault Locators (VFL): These are specialized tools that inject a visible red laser (usually 635nm - 650nm) into the fiber specifically for troubleshooting breaks. This is the only time you would see “colorful” light (red) leaking from a fiber.

For more technical details on how light interacts with our sensors, you can refer to the OFSCN® Fiber Bragg Grating (FBG) Technology Overview.