Hans Poisel

Comparing polymer optical fiber, fiber Bragg grating, and traditional strain gauge for aircraft structural health monitoring

Systems for structural health monitoring in aeronautical structures use methods of measuring the elongation that normally require too heavy setups or difficult assembly jobs, such as those based on traditional strain gauges. Alternative methods based on fiber Bragg gratings tend to be very expensive. We analyze the possibility of improving the existing designs with the aid of low-cost plastic optical fiber sensors. For this purpose we test these sensors in a rudder flap subjected to different types of bending forces. The results show that they offer good stability and repeatability, and the measured values are very similar to those obtained with Bragg sensors.

Passive fiber optic gyroscope.

A fiber optic gyroscope different from the standard concept is presented. A fused fiber 3 x 3 directional coupler provides a constant phase shift thus enabling the detection of rotation rate at the quadrature point without phase modulation. Bias errors due to birefringent coupling centers in the fiber coil are avoided by using an unpolarized light source. A contrast insensitive signal recovery scheme eliminates the influence of polarization fluctuations on the scale factor. First measurements with a prototype gyroscope (90 mm in diameter and 23 mm in height) show a bias stability of <4.7 degrees /h and scale factor accuracy of <0.1% in the range of +/-200 degrees /s.

Low-cost passive fiber optic gyroscope

This paper presents a passive fiber optic gyroscope without nonreciprocal phase modulation. A fused fiber 3 X 3 directional coupler provides a constant phase shift, thus enabling the detection of rotation rate at the quadrature point. Bias and scale factor errors due to indeterministically changing birefringent coupling centers in the non-polarization preserving monomode fiber coil are eliminated by using a contrast insensitive signal recovery scheme. A simple temperature compensation procedure has been implemented in the gyro software which yields a bias stability < 0.04 deg/sec and scale factor errors < 0.5% in the whole temperature range between -40

Fiber-optic depolarizer

DEGC and +70 degree(s)C.