Low-coherence Multiplexing For Distributed Fiber-optic Sensor Systems

Sensors, which measure physical quantities and convert them into readable signals, are of great importance to human beings in the Information Age. Fiber-optic sensors, due to their specific advantages, have attracted much research attention in the last 40 years, together with the significant development of fiber optics since 1960s. Many fiber-optic sensors are possible being connected by communication fibers and forming a sensor network, in which a multiplexing scheme is the key technique that helps to distinguish signals from different sensors. One cost-effective multiplexing technique, named low-coherence multiplexing for using low-coherence light source, were studied in details with great research interests in the Ph.D. project. Equal attention was paid to design and develop novel sensors fitting for such a multiplexing scheme. Both form a hard-core basis of this thesis.Fiber-grating-based interferometers are described in Chapter 2. They are extremely interesting for our work as they keep both properties of fiber grating, one kind of sensitive sensor, and advantages of interferometer, which, for instance, can be interrogated by a low-coherence multiplexing system. These compact in-fiber interferometers were fabricated in the lab, by a home-made system with equipments under fully automatic control. The author took main part in the construction of the system and technical details are given in this chapter.Due to the absence of effective multiplexing scheme, long-period-fiber-grating (LPG) sensors have difficulties being used in the sensor network, until our first successful demonstration of using the low-coherence multiplexing method. This breakthrough is discussed in details in Chapter 3. The performance of the multiplexing system is tested by measuring bending, temperature, and refractive index of liquid, which is of great interests for bio-analysis. The low-coherence multiplexing technique is proved to be also valid for fiber-Bragg-grating (FBG) sensors in Chapter 4, by constructing pairs of FBGs into Fabry-Perot interferometers. The sensitivity of the sensor can be dramatically increased if measure the mismatch between the grating pair instead of detecting the wavelength of either FBG directly. Such a technique was used for Methane measurement, by transferring the concentration information into temperature through some catalysts.The low-coherence multiplexing schemes for absorption sensor and Faraday sensors are explained in details in Chapter 5. Particularly, the compatbility of the technique to these two types of traditional and widely-used sensors are emphasized.Finally in Chapter 6, a novel low-coherence optical frequency domain reflectometry (LC-OFDR) is presented. Different from a normal OFDR, the maximum measurable distance of such a reflectometry is not limited by the coherent length of the light source. Moreover, LC-OFDR resolves ranges in frequency domain, with a much fater speed compared with slow mechanical scanning, that used by the low-coherence reflectometry described in Chapter 3 to 5. In principle, LC-OFDR can upgrade all the low-coherence multiplexing systems discussed in this thesis.A general conclusion is drawn in Chapter 7, and future works are looked forward.