Oscillating optical fiber speckle patterns: modeling and application

Abstract

Speckle patterns produced by the interference between multiple optical fiber modes carry detailed information regarding the waveguide characteristics. However, reflection-type setups implemented with fiber coupler and mirror generate oscillating specklegrams that cycle consistently around well-defined states. To study such a peculiar phenomenon, an analytical model derived from a multimode fiber Fabry-Pérot interferometer investigates the contributions of the cavity length, refractive index, and laser wavelength, revealing a sinusoidal modulation of the correlation coefficient. Experiments confirm the effect of the fiber-mirror distance on the specklegram changes: computing the correlation and tracing the spatiotemporal evolution of a light granule through active contour models yielded a phase sensitivity of 15.71 rad/µm. Furthermore, the presented system works as a precise displacement sensor enhanced by the extended correlation algorithm, achieving 3 nm resolution with linear response over a 70 µm range.

Agar-based optical sensors for electric current measurements

“…Albeit discerning both effects is not straightforward, Fig. 3 suggests that the speckle granules consistently follow the current flow, i.e., tracing their displacement rate through image registration and active contouring algorithms 50 yield an estimation of the electrical stimulus regardless the agar volumetric changes.…”

Recent developments in agar-based optical devices

“…Despite their apparent chaotic and random behavior, specklegrams encode detailed and consistent information about the fiber state [56]. Since disturbances from physical or chemical stimuli modulate the speckle field by mode coupling and phase deviation effects [57], one may acquire the output specklegram and evaluate its spatiotemporal changes to predict the input variable. Notably, this approach demands a visible laser and a camera instead of expensive broadband sources and spectrometers, besides achieving distributed response and high sensitivity, being comparable to interferometric systems [58].…”