Fiber pH sensor based on long period gratings

A fiber PH sensor based on an air-gap long period fiber grating (AG-LPG) in a multi-modes fiber is proposed with the sensitivity of -0.219 nm/pH and 0.248 nm/ pH for measuring the acid and alkaline solutions respectively. The operating mechanism is based on the index of different pH-value solutions to cause the grating loss-dip wavelength shift due to the change of the effective index both in core and cladding layers. This simple sensing head can be applied in a long-term monitoring pH change of drinking water or testing solutions.