Completed Research 1995 to 2006

Theme 2 (1999-2002):
Fibre Optic Sensor Technology

PROJECTS IN THEME 2

As part of the 1998 NCE mid-term review of the ISIS research program, Theme 2 was renamed to better reflect the scope of activities taking place within the theme.

Fibre optic sensors are well suited for providing intelligent structural sensing for civil engineering structures. One important class of structures that can greatly benefit from this technology is that of bridges, where it will be possible to centrally monitor a large number of bridges instrumented with FOSs. On site inspections will only be necessary when this remote sensing system indicates potential problems have developed such as excessive loads, structural failure and environmental degradation.

There are two types of FOSs which can be used for monitoring purposes, Fabry Perot (FP) gauges and fibre Bragg gratings (FBG). Each sensor is based on a different fibre optic sensing principle and provides specific benefits depending on the application. These sensors can be used to monitor various structural responses such as: static and dynamic loads from traffic, high winds and earthquakes; long term corrosion damage to reinforcements and concrete; strain relief in prestressed tendons; interfacial strain between FRP repair patches and the concrete structure; and internal strains and fractures in the FRP and concrete and their cure state during construction.

In this research program, four sensing modalities were investigated for civil engineering structures, with particular attention to bridge applications. These are: short gauge length Bragg gratings and FP sensors (mm to cm), long gauge length Bragg grating strain sensors (cm to m), serial and parallel multiplexed Bragg grating and FP sensor arrays, and extended Bragg grating distributed strain sensors. Generic to all integrated fibre optic structural sensing are the issues of installation, interfacing and connectivity.

This research was particularly relevant to the embedding of fibre optic grating and FP sensors within prestressing, or post tensioning, tendons and other reinforcements during their manufacture. A study of the influence of the pultrusion process on the performance and functional life of the embedded grating and FP sensors was undertaken.

Research was also carried out on serial multiplexing of Bragg grating sensors that are formed along a single optical fibre. In particular, a comparison was made of the relative merits of wavelength and time division multiplexing for this task. Special effort was devoted to developing an interrogation system which can handle as many as 20 gratings on the same fibre string. These sensors were monitored using an ultra short optical pulse. The short duration of the pulse allows numerous sensors to be multiplexed in time along the same fibre using a time of flight criterion, allowing each sensor to be designed at the same nominal wavelength. Each sensor can be de multiplexed using optical or electronic grating.

A long gauge structural sensor can provide integrated strain measurements that are not susceptible to the high local strains associated with crack forming in concrete. This type of sensor is also well suited for incorporation within rehabilitation and strengthening wraps. The relative merits of nonlinear and low coherence approaches were investigated to determine which is better suited for the construction industry.

The feasibility of combining short and long gauge length sensors in the same optical fibre using Bragg gratings for both types of sensor was also investigated, and methods of temperature compensation for these sensors developed. Parallel multiplexing of both short and long gauge FBG sensors was studied. To this end it is essential that a multi channel, low cost, compact, rugged and reliable fibre optic strain sensing system for field and laboratory applications be developed. In this program, the goal was to complete the design and develop microchip integrated optoelectronic demodulation systems for application to the fibre optic sensing systems being developed.

The heart of this system is a tunable distributed feedback laser (DFB) which employs semiconductor diode lasers as the light source. This application requires the fabrication of lasers emitting light at a well defined wavelength (specific to the application) and with a very narrow line width that can be controllably tuned over a wavelength range of l 0nm. In order to meet all the requirements of the various applications the preferred type of laser is fabricated using a strained layer quantum well structure grown using the (InGa)(AsP) materials system. This allows the fabrication of lasers emitting in the wavelength range 1550nm. Thus, this project constituted a vital link between materials growth and fabrication aspects and the integration of completed devices in sensor detector arrangements.

A new approach was pursued in which integration of the DFB laser and a non absorbing distributed Bragg reflector (DBR) to provide the required lasing wavelength, controlled by the injection current into the laser and DBR. Hence, developing the enabling technology formed an essential part of the Theme 2 research program to utilize fibre optic sensors in civil engineering structures.

Throughout the entire program of Theme 2, ISIS Canada researchers worked with a number of private sector organizations. The potential economic benefits of this Theme was two fold. One was the enhanced ability that the research results would provide to those managing infrastructure to monitor remote facilities at low cost. The other was the potential for spin off companies that could export this technology throughout the globe.

Completed Research 1995 to 2006