Cladding Modes

What are Cladding Modes?

Cladding modes in an optical fiber (or other kind of optical waveguide) are modes the intensity distribution of which is not restricted to the region in or immediately around the fiber core. Although one often does not intentionally feed light into cladding modes, these play an important role in fiber optics.

When a properly focused and aligned laser beam hits the end face of a fiber, most of its power may then propagate in the fiber core. Some fraction of the power, however, will propagate in cladding modes. Depending on the refractive index of the surrounding fiber coating, one can have two different situations:

• The most common case is that the coating has a higher index than the cladding — for example, for the common acrylate or polyimide coatings on silica fibers. Here, light in the cladding can fully get into the coating, where it experiences substantially increased propagation losses. The strong attenuation of cladding modes can be very convenient, e.g. when the launch efficiency for the core is optimized by changing the alignment of the fiber or some coupling optics: a photodetector behind the fiber will then monitor only light launched into the fiber core.
• If a low-index coating is used, e.g. made of a fluorinated polymer, cladding modes stay confined to the cladding, reaching only slightly into the coating (with exponential decay of intensity). Losses of cladding modes can then be fairly low. This is exploited for many double-clad fibers and in a few other occasions, e.g. for cladding-mode fiber sensors.

When working with rather short pieces of fiber, e.g. to measure some strong absorption in a highly doped rare-earth-doped fiber, incomplete elimination of power in cladding modes may be a problem. This is particularly the case when the polymer coating around the cladding has to be removed. A possible solution is to splice the short fiber to a longer length of passive fiber, which serves to eliminate light in cladding modes. Another possibility is to use a droplet of index-matching fluid on the fiber. For high-power fiber lasers and amplifiers, there are special cladding mode strippers which can remove cladding light at high power levels.

Due to their different propagation constants, core modes and cladding modes usually do not exhibit any noticeable coupling with each other. This means, e.g., that light launched into the core will remain there and not leak into cladding modes, and vice versa. However, such leakage may occur if there is a deviation from the regular core and cladding structure — particularly if there is a periodic disturbance, the period of which is matched to the differences of propagation constants. This effect is utilized in long-period fiber gratings, which cause loss in some wavelength range by coupling light from the core into the cladding modes.

Propagation of pump light in an inner cladding (pump cladding) is often used for high-power fiber lasers and amplifiers based on double-clad fibers. Here one exploits the fact that it is much easier (and requires a much lower beam quality) to launch light into a multimode cladding. The pump light can still be absorbed in the core because most cladding modes have some overlap with the core region.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains six suppliers for cladding mode strippers. Among them:

⚙ hardware

Our cladding power strippers are used in high-power fiber lasers and amplifiers which need to handle substantial optical powers. The devices ensure that these powers are absorbed in a sufficiently widespread region, and that the generated heat can be removed safely, without damaging the mode stripper or any surrounding parts. We can provide them with all kinds of double-clad fiber and for cladding powers up to 500 W.

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