Multiplexing and transmission of RF signals using an optical fiber

Ultrasonic non-destructive testing systems designed to control huge structures normally use several transducers in the reception stage. To avoid increasing the cost of electronics, a multiplexer is used to send all received signals to the same processing module. Traditionally, transmission of such signals is carried out using copper cables. For special applications (i.e. continuous monitoring of nuclear plants) metallic cables are not suitable because of their high sensitivity to electromagnetic perturbations. Moreover, the multiplexing is made electronically. When the distance between the transducers and the reception unit is large and/or electromagnetic noise is important, signal degradation takes place. The proposed system implements the transmission and multiplexing of ultrasonic electrical signals obtained by means of broadband transducers (up to 1 MHz), using an optical fiber. Optical fibers are made of dielectric materials (silica or plastic) so they are inherently passive to electromagnetic noise. Wavelength division multiplexing is utilized for adding channels to the system by means of fiber optic couplers and different light sources. The wavelengths of the optical signals utilized are located far apart in the optical spectrum in order to avoid serious crosstalk in transmission. The limit to the number of multiplexed channels depends on the optical fiber selected, the spectrum of the light sources and the wavelength division multiplexers or couplers utilized.     

Optical Communications in the Universidad Publica de Navarra

In this article, two main resarch lines in photonics carried out by the optical communications group of the Universidad Publica de Navarra are shown. The article is focused on WDM subsystems, microwave and photonics, smart structures, and fiber-optic sensors development.     

Ultralong 250 km remote sensor system based on a fiber loop mirror interrogated by an optical time-domain reflectometer

A 253?km ultralong remote displacement sensor system based on a fiber loop mirror interrogated by a commercial optical time-domain reflectometer is proposed and experimentally demonstrated. The use of a fiber loop mirror increases the signal-to-noise ratio, allowing the system to interrogate sensors placed 253?km away from the monitoring system without using any optical amplification. The displacement sensor was based on a long period grating spliced inside of the loop mirror, which modifies the mirror reflectivity accordingly to the applied displacement.     

Double random mirror Hi–Bi photonic crystal fiber Sagnac based multiwavelength fiber laser

In the present work, a multiwavelength Raman fiber laser based in a Sagnac configuration is presented. The Sagnac configuration is composed of a Hi–Bi photonic crystal fiber in the middle of two dispersion compensation fibers. The pump power induces two random mirrors to work in opposite directions in the Sagnac, leading to the amplified signal and the multiple scattering signals from the random mirror traveling through the Hi–Bi photonic crystal fiber in a counter-propagating way interfering in the output port.     

High precision micro-displacement fiber sensor through a suspended-core Sagnac interferometer

A sensing system for micro-displacement measurement based in a suspended-core fiber Sagnac interferometer is presented. The suspended-core fiber characterization was made through the use of an optical backscatter reflectometer, screening its multimodal and birefringent behavior. Its sensitivity to displacement measurements is shown to be due only to birefringence, being that core-cladding mode coupling is negligible. High precision (～0.45 μm) was obtained using three different measurement instruments, showing an extremely high stability and high insensitivity to temperature, demonstrating that the sensing system has the ability for low cost applications.     

Multiwavelength fiber laser based on a photonic crystal fiber loop mirror with cooperative Rayleigh scattering

In this work, a multiwavelength fiber Raman laser based on a highly birefringent photonic crystal fiber loop mirror is presented. A laser resonator is formed when the Raman amplification with cooperative Rayleigh scattering in a dispersion-compensating fiber is used as a distributed mirror and combined with a photonic crystal fiber loop mirror filtering structure. Stable multiwavelength lasing at room temperature is achieved due to the low temperature sensitivity of the highly birefringent photonic crystal fiber.     

Simple low-loss waveguide bends using ARROW effect

A new concept for reducing bend loss in dielectric planar waveguides is presented. It is based on the introduction of a set of antiresonant reflecting optical waveguides (ARROWs) on the outside of the bent core and defined in the same fabrication step as the main bend. It has been ascertained by simulation that the bending loss can be significantly reduced. The method is compatible with other bend-lossreduction strategies, as well as with different waveguiding structures, such as rib and buried waveguides and fibers.This revised version was published online in August 2005 with a corrected cover date.