We can shortly define the benefits of the fiber optical technology as compared with the classic cable type temperature detection systems used in the fire alarm systems as the ability to detect at longer cable distances and sensing minor temperature changes. Detection of temperatures at distances up to kilometers, particularly in highway and rail tunnels, is quite difficult in classic systems. On the other hand, fiber optical temperature detection systems allow for the detection of even the smallest temperature increases (below 1°). In addition, with this system, distributed detection is possible at very long distances such as conveyors, cable channels, highway and rail tunnels etc.
Fiber optical temperature detection is a type of optomechanical linear heat detection. For a single detection system consisting of an optical fiber and a detector unit; the cable may be several kilometers long. The detector unit emits a pulse of light onto the optical fiber and detects the reflected light returning from the fiber optical cable. The intensity or wavelength of the reflected light, along with the time between emitting the light pulse and detecting the reflected signal, provides information about the temperature along the fiber optical cable. It is possible to continuously monitor the temperature distribution along the entire cable and to follow the temperature changes at the time of a fire.
There are some different technologies for fiber optical temperature detection. The most common techniques for distributed temperature detection are: Raman scattering, Brillouin scattering and Fiber Bragg Gratings (Fbg) and among which Raman scattering is the most common. Both Raman and Brillouin scattering uses the temperature dependence of light scattering due to molecular vibrations within the glass core of the optical fiber. Fiber Bragg gratings reflect back a certain wavelength of the incoming light. Due to the parameter to be measured (temperature, pressure), the wavelength of the reflected light changes as a result of the change in the grating period. The gratings (temperature detection points) may be at the intervals of each meter along the cable.
