A low-cost temperature sensor based on long period fiber gratings with linear wavelength transmission characteristics

The discovery of photosensitivity in germanosilicate fibers and the invention of the side writing technique has made fiber Bragg gratings (FBG) suitable as narrow band filters in fiber optics communications. In parallel, Bragg gratings also find wide applications in the area of sensing systems, such as temperature and strain sensors. But from the past temperature sensing experiments, we found that Bragg gratings suffer from the limited temperature sensitivity (similar to 0.04nm/degrees C at 1550nm) and often require complex and expensive interferometric techniques to detect measurand-induced wavelength shifts. On the contrary, long period fiber gratings (LPG) have attracted many interests for temperature sensing systems owing to their superior thermal sensitivity. It has been demonstrated that a long period fiber grating has the thermal sensitivity 5-10 times greater than that of a FBG(1,2). However it is difficult to accurately measure the center wavelength of a LPG because of its large bandwidth(3,4). Besides, since the LPG has no reflection spectrum, it would require additional instrumentation compared to FBG interrogation techniques. Vensarkar et al. have used a narrow linewidth light source centered at one side of a LPG's peak loss wavelength and a photo-detector to measure the varying transmission power(5). They found an approximately linear relationship between temperature and transmission power in logarithmic scale. In this paper, however, we demonstrate a novel scheme to measure the transmission power in linear scale using a hybrid sensor consisting of a FBG and a narrowband LPG(6).