Statistics of modal noise in fibers: a case of constrained speckle.

Modal noise in multimode optical fibers is usually analyzed by using the conventional statistical theory developed for speckle. We show that when little light is lost from the fiber to radiative modes, a constraint of constant total power is introduced that is not present in conventional speckle theory. Experimental results show a marked departure in the behavior of speckle signal-to-noise ratio from the predictions of conventional speckle theory. A suitable modification of this theory is introduced, bringing the theoretical and experimental results into agreement.