A novel fiber chemical sensor using inner-product multimode fiber speckle fields

Although multimode fiber has a large space-bandwidth product (roughly equal to the number of modes), transmitting spatial signals by using a fiber is rather difficult. Conventional sensing schemes for fiber chemical sensors are based on the intensity and/or spectroscopic absorption or emission of fiber evanescent field of multimode fiber. However, the transmission of spatial signal has not been exploited in the field of chemical sensing. In our research, a novel chemical sensing method that employs the spatial information of a multimode fiber was investigated. When a laser beam is launched into a multimode fiber, the exit light field produces a complicated speckle pattern caused by the modal interference of the fiber. It is difficult to recover the transmitted information from the speckle field. However, the fiber speckle field can be used for fiber sensing with an inner product method. Our analysis and experiments have shown that the fiber specklegram sensor is very sensitive to chemical environment change.