Photonic lanterns are made by adiabatically merging several single-mode cores into one multimode core. They provide low-loss interfaces between single-mode and multimode systems, where the precise optical mapping between cores and individual modes is unimportant.

The photonic lantern

A taper transition can couple light between a multimode fiber and several single-mode fibers. If the number of single-mode fibers matches the number of spatial modes in the multimode fiber, the transition can have low loss in both directions. This enables the high performance of single-mode fiber devices to be attained in multimode fibers. We report an experimental proof of concept by using photonic crystal fiber techniques to make the transitions, demonstrating a multimode fiber filter with the transmission spectrum of a single-mode fiber grating.

/pdf/multimode-fiber-devices-with-single-mode-performance-2njua7brsf.pdf

Multimode fiber devices with single-mode performance.

Bragg gratings in optical fibers in multimode propagation are investigated experimentally and theoretically. Bragg gratings formed in optical fibers in multimode propagation show multiple reflection peaks or multiple transmission dips in the reflection or transmission spectra, respectively. For standard graded-index multimode fiber, the number of reflection peaks of a Bragg grating depends on excitation condition of propagating modes. The number of reflection peaks of a Bragg grating at around 1.55 /spl mu/m is 19 for highly multimode excitation and 3-4 for lower order mode excitation. We analyze the phase-matching conditions of the propagating modes and identify half of the reflection peaks as the reflection to the same mode and the rest as the reflection to the neighboring modes. In dispersion-shifted fiber, a Bragg grating at around 0.8 /spl mu/m in three-mode propagation shows three reflection peaks in the reflection spectrum. The temperature dependence of each reflection peak is similar to that of a conventional Bragg grating in single-mode fiber. Polarization dependence measured on a Bragg grating in multimode graded-index fiber is negligible. An advantage of Bragg gratings in multimode fiber (MMF) and the applications are discussed.

Bragg gratings in multimode and few-mode optical fibers

We report for the first time the inscription of retro-reflective Bragg gratings in multimode crystalline sapphire fiber. The gratings were fabricated using 800-nm femtosecond laser radiation and a phase mask. The grating behavior was investigated up to 1500/spl deg/C with no detectable reduction in the grating reflectivity or hysteresis in the Bragg resonance. Measurements of the change in the effective index of the fiber as a function of temperature are reported and the performance of the grating as a temperature sensor is evaluated.

Sapphire fiber Bragg grating sensor made using femtosecond laser radiation for ultrahigh temperature applications

We describe use of swept-wavelength interferometry for distributed fiber-optic strain and temperature sensing in single mode and gradient index multimode fiber. The method is used to measure strain in a four-strand multimode cable under twist.

T. Niiho

Papers

Bragg gratings in multimode and few-mode optical fibers

Multimode Fiber Bragg Gratings for Fiber-Optic Bending Sensors