Light emitted by fiber optic sensor

Fiber optic sensors typically emit infrared light, often generated by LEDs or laser diodes, with common wavelengths around 1310 nm and 1550 nm, though visible light can be used for short-range applications.

Light Type and Wavelength

Fiber optic sensors use light as the sensing medium, which can be either infrared or visible light depending on the application. For most industrial and communication-grade sensors, infrared light is preferred because it experiences lower attenuation in silica fibers, allowing signals to travel longer distances with minimal loss. The most common infrared wavelengths are 1310 nm and 1550 nm, which correspond to the low-loss transmission windows of optical fibers, ensuring efficient signal propagation and minimal distortion . For short-range or low-cost applications, visible red light (~650 nm) may be used, but it suffers higher attenuation and is generally limited to distances under 50 meters .

Light Sources

The light in fiber optic sensors is typically generated by semiconductor laser diodes or light-emitting diodes (LEDs). Laser diodes provide highly coherent, narrow-spectrum light suitable for long-distance or high-precision sensing, while LEDs are cost-effective and sufficient for shorter-range applications . The emitted light is coupled into the fiber, guided by total internal reflection, and interacts with the sensing environment either directly (through-beam) or via reflection (reflective sensors), .

Interaction with the Fiber and Sensing

In extrinsic fiber optic sensors, the fiber acts as a conduit to transmit light to and from an external sensing element, while in intrinsic sensors, the fiber itself is the sensing element, and the light's properties—such as intensity, phase, or wavelength—are modulated by the physical parameter being measured, like strain, temperature, or pressure . The sensor detects changes in the light after it interacts with the object or environment, allowing precise measurement without electrical interference.

Summary

• Primary light type: Infrared (1310 nm, 1550 nm), sometimes visible red (~650 nm)
• Light sources: Laser diodes (high precision, long distance) or LEDs (short-range, cost-effective)
• Function: Light travels through the fiber, interacts with the environment, and is analyzed to detect changes in intensity, phase, or wavelength . This combination of light type and fiber guidance enables highly sensitive, non-contact, and interference-resistant sensing in industrial, medical, and communication applications.