What makes fiber optic sensors emit light

Fiber optic sensors typically emit visible or infrared light, often using LEDs in red, blue, pink, or infrared wavelengths depending on the application.

Types of Light Sources

Fiber optic sensors use light-emitting diodes (LEDs) or sometimes laser sources to generate light that travels through the optical fiber to the sensing area. Common wavelengths include:

• Red light (around 633 nm): Offers high process stability and is effective for detecting bright or white objects .
• Blue light (around 455 nm): Suitable for precise measurements on dark, glossy, or glowing surfaces due to its shallow penetration .
• Pink light: A combination of red and blue LEDs to increase light output and improve sensor range .
• Infrared light (over 750 nm): Invisible to the human eye, ideal for applications where visual distraction must be avoided, and provides longer sensing range .

How the Light is Used

The emitted light interacts with the target object in different ways depending on the sensor type:

• Through-beam sensors: Light travels directly from the emitter to a receiver; interruption by an object triggers detection .
• Reflective sensors: Light reflects off the object back to the receiver, with intensity changes used to detect the object .
• Retro-reflective sensors: Light is reflected back from a reflector, and object interruption reduces the received light .

Modulation for Measurement

In addition to simple detection, fiber optic sensors can measure physical quantities by modulating the light's intensity, phase, or wavelength. For example:

• Intensity modulation: Detects changes in light amount due to object presence or micro-bending .
• Phase modulation: Used in interferometric sensors to detect strain or temperature changes .
• Wavelength modulation: Seen in Fiber Bragg Gratings (FBGs), where strain or temperature shifts the reflected wavelength for precise measurement .

Summary

Fiber optic sensors emit visible or infrared light, primarily from LEDs, which is transmitted through optical fibers to detect objects or measure physical parameters. The choice of wavelength and modulation method depends on the application, surface properties, and required measurement precision .