Cladding Modes - Definition and Applications

Cladding modes are the regions, or modes, in optical fibers that are not restricted to the region around the core of the fiber. More specifically, cladding modes are regions in which the intensity of distribution is not restricted to the region in and around the fiber core.

These modes are significant in the construction of fiber optic sensors. The cladding mode is more confined to the cladding, due to its high refractive index. Cladding modes are not desirable in various fiber optic applications.

While constructing fiber optic sensors, the optical cables need to incorporate either a double-clad fiber or an additional polymer coating, in order to overcome the cladding modes.

Basic Principle

Cladding modes are also called radiation modes. When an aligned laser beam enters a fiber, some faction propagates through cladding modes. Depending on the coating that surrounds these modes, they may either propagate light over long distances, or attenuate it.

Double cladding fiber (DCF) consists of a core, inner cladding, and an outer cladding. The refractive index of the core and the outer cladding is higher than that of the inner cladding. The cladding mode is excited to resonance by wave coupling, in order to measure the refractive index of the medium.

In high power applications, cladding mode strippers are used to remove the cladding light at high power levels.

Applications

Some of the applications of cladding modes are listed below:

• Refractive index sensors
• CO₂ sensors
• Chemical fiber optic sensors

References

• Properties and Sensing Applications of Long-Period Gratings – Virginia Tech
• Fiber Optic Refractive Index Sensor based on Cladding mode resonance - Academia
• Cladding Modes – RP Photonics