Massively Degenerate Coherent Perfect Absorption in Gradient‐Index Fibers

Coherent perfect absorbers (CPAs) enable even weakly absorbing materials to achieve complete absorption by exploiting interference. Generalizing this to arbitrary multimode input states remains challenging, because conventional implementations work only for a single or very few input channels. Here, a compact multimode coherent perfect absorber based on a gradient‐index (GRIN) fiber is proposed, in which the self‐imaging property of the fiber replaces the bulky free‐space architecture of previous approaches. Unlike a free‐space self‐imaging cavity, a realistic fiber cannot be exactly degenerate, raising the question of how much modal degeneracy such a device requires. An analytic relation is derived linking the residual modal phase spread directly to the field‐of‐view reflectivity. Absorption survives as long as the modal dephasing remains small compared with the coupling bandwidth of the cavity, so that residual dephasing imposes an upper bound on the usable cavity finesse – a design constraint absent in the free‐space case. Applying this criterion shows that GRIN profiles optimized for minimal intermodal group‐delay dispersion, a condition derived for pulse transport, simultaneously optimize monochromatic modal rephasing, so that commercial graded‐index fibers reach field‐of‐view reflectivities well below . This offers a scalable route toward multimode absorption in fiber‐based and integrated photonics.