Intermodal Interference Research Articles

Hot spots and supermodes: The theory of ambient sound in the ocean‐surface bubble layer

An elegant series of experiments was conducted recently by Farmer and Vagle [J. Acoust. Soc. Am. 86, 1897–1908 (1989)], in which the sound from individual wave‐breaking events was recorded by a hydrophone located a few meters beneath the ocean surface. Between 1 and 20 kHz, the observed spectra from La Perouse Bank contain relatively sharp, isolated peaks (hot spots) at intervals of approximately 3 kHz, whereas the data set from the FASINEX experiment shows energy spread over broad spectral bands (supermodes), perhaps 3 kHz wide, and separated by fairly well‐defined energy gaps (i.e., spectral minima). Both types of spectrum are a manifestation of waveguide propagation in the upward refracting, ocean‐surface bubble layer. A new, modal theory of sound in an upward refracting duct shows that, in general, the mechanism responsible for the observed hot spots and supermodes is intermode interference. The qualitative differences between the two data sets arise because the number of modes supported by the profile at La Perouse is significantly less than the number in FASINEX. In both cases, the detailed features displayed by the theoretical and experimental spectra show an almost one‐to‐one correspondence. [Research supported by ONR.]

Application of intermodal interference to fibre sensors

Interference of the fundamental mode with LP 11 and LP 02 are used to separate temperature variation from longitudinal strain in a fibre sensor; the measurement technique is fringe counting. Polarisation fading is overcome by triple detection through different analysers.

Optical interference and pulse propagation in multimode fibers

Abstract The role played by optical interference in determining the propagation of pulses in multimode fibers is examined under conditions of mode independence and mode coupling. Single pulses and pulse-pairs generated by a source that has arbitrary temporal and spatial coherence are considered, and the role played by the spatial integration at the detector, which effectively is a form of spatial incoherence, is investigated. It is shown that three forms of optical interference may result under different conditions of source spatiotemporal coherence—intramodal, intermodal, and interpulse interference. These effects change the profile of received pulses in a, in general, nonlinear way.