A twin-core fiber based long period fiber grating (LPFG) has been proposed and fabricated by using CO<sub>2</sub> laser pulses
exposure approach. It makes two long period gratings in parallel and integrates the two fiber LPFG devices in one fiber.
It could be used in biological, chemical and environmental sensing system.
Two-core fiber is a specially designed fiber which contains a pair of parallel fiber core surrounding a cladding with
diameter 125μm. A multi-parameters measurement technique that uses a two-core fiber as the sensing element has been
proposed and demonstrated in this paper. The theory and the structure design are also introduced. The two-core fiber acts
as a two-beam interferometer, in which phase differences is a function of curvature, and the twisting angle in the plane
containing the cores results in the shift of the far-field interferometric fringe pattern. This sensor can be used to inspect
the structural health monitoring, the measuring angle and the distinguishing direction. Compared to the traditional
multi-beam interferometer, this sensor has the such characteristics as small size, good interference, high definition and
steady fringe pattern. It does not need considering the effect of the environment temperature. A low-coherence laser
diode at wavelength 650nm illuminating the two-cores and the interferogram pattern in the far-field is recorded by a
CCD camera. The model of two-core fiber sensor has been established theoretically. The relationship between the
far-field fringe pattern intensity distribution and the changes due to the radius of the curvature and the twisting angle are
given, and the experimental results also confirmed this.
A multi-parameters measurement technique that uses four-core fiber as the sensing element has been proposed and
demonstrated. The four-core fiber act as a four-beam interferometer in which phase differences is a function of
curvature, twisting angle, and the variation of temperature in the plane containing the cores results in the shift of the farfield
interferometric grid pattern. The relationship between the far-field grid pattern shift and the multi-parameters has
been established and the experimental results were also confirmed this.
Multi-core fiber means two or more single mode core embedded in one cladding. In this work, a special four-core fiber
has been designed and demonstrated, in which, the internal distance between the neighbor cores is 18 micrometer. The
specially designed four-core fiber can be used as optical grid pattern generator or fiber optic sensors. A low-coherence
LD light source has been used in the experiments. On the basis of the intensity Gaussian distribution of each individual
single mode core, the far-field grid pattern is obtained and it is in good agreement with experimental result.