There is a range of ways to couple light in a single mode fiber (SMF) from core mode to cladding modes, which can be
applied in some fiber sensors. Recently, a very simple method using CO2 laser irradiation is put forward. By coupling
core mode to cladding mode in the first irritation point and re-coupling in the second one, in-line Mach-Zehnder
interferometer (MFI) and Michelson interferometer(MI) sensors have be demonstrated.
To understand the mechanism underneath this coupling phenomenon, several parameters (laser power, laser lasting time,
etc) tests are investigated. With bigger laser power and longer lasting time, one can obtain higher mode coupling, which
is potential for greater sensitivity sensor. Combined with a long period fiber grating (LPFG), the cladding modes
promoted in fiber cladding are studied.
In some big power conditions, permanent deformation can be met on the irradiation points of the fiber. Although higher
loss is induced, there is also other advantage, such like high temperature stability. The sensitivity and stability of
temperature are discussed based on these sensors' configuration and mechanism. The experiences verify our laser
irritation sensors can survive in very high temperature. When coating with some gas absorption film such like zeolite
film, one reliable high sensitivity gas sensor is successfully demonstrated in low ppm vapor level.