Fiber optic sensors exploit the interaction of light with environmental parameters to be sensed (measurands) such as temperature, pressure, flow, strain and vibration For each conventional electrical sensor such as a thermocouple and strain gage, there exists a fiber optic analog of similar size and weight, where the measurand might modulate the optical power, phase, wavelength or polarization Fiber optic sensors are attractive for subsea applications for several reasons
The electrical power source can be remotely located No electrical power is required at the sensing point
Multiple sensors can be placed in a single optical fiber to detect multiple measurands along the fiber length Multiplexing techniques developed for the fiber optic telecommunications industry can be used to demultiplex the signals at a remote location
Although much work has been done on fiber optic sensor technology over the last twenty years, many of the sensing devices are still developmental Other devices have been commercialized and are cost competitive with conventional electrical sensors Both developmental and commercial fiber optic sensors are discussed with emphasis on potential fiber optic sensor configurations for future subsea use Potential applications include strain measurement on risers and sensors for remote monitoring and control instrumentation on subsea completions Practical constraints and technical challenges for fiber optic sensor technology are also discussed
Specialty fiber optic sensors continue to be developed for many industrial and engineenng applications Test and demonstration projects have been conducted over the last several years Applications are niche-driven and tend to be in those areas where fiber optics offers distinct advantages Examples include pressure measurements in 400°C reactors and temperature measurements in high voltage transformers However, the sensor industry as a whole is primarily driven by special applications for specialty sensors Little universality is present in this industry Thus, fiber optic sensors are simply on more technology among the collection of sensor and transducer technologies, each of which is engineered into special products for special customer needs
Of the many individual fiber sensor types (generically categorized as interferometric, polarization, wavelength, and intensity) demonstrated in the lab, fiber intensity sensors are most advanced from an engineering and commercial standpoint Optical and fiber optic intensity sensors predominate in industrial applications for several reasons These include
simple, manufacturable sensor assemblies
simple, low-cost signal processors
available self-referencing methods to significantly reduce link sensitivity
compatibility with optical time domain reflectometry methods for sensor multiplexing.
In the past, these factors have favored intensity sensors for commercial applications However, other sensor types, especially fiber Bragg grating sensors show extremely good promise for some commercial applications in the petrochemical industry
In this paper, examples of three types of fiber optic sensors are described--intensity, time-rate-of-decay, and wavelength Multiplexing methods and subsea application areas for these sensors are discussed
An advanced prototype fiber optic pressure transducer has been developed for use as a gage pressure transmitter in process applications and as an absolute pressure transducer in aircraft applications
