Advantages for sensor applications of long-period gratings (LPGs) in special optical fibers are reported. Two consecutive LPGs separated by 60 to 100 mm interfere to improve the resolution and reduce noise in a highly doped fiber with inner cladding and in a D-shaped fiber. These gratings provide good contrast to increase the resolution for sensing applications, with or without access to the surroundings along the fiber. The mode profiles of the devices were characterized experimentally to gain deeper insight into the improved functionality.
Regenerating at constant temperature under load allows inelastic changes in glass that can be exploited to tune the Bragg wavelength of a filter to any arbitrary spectral position. We have reported < 20nm of tuning of a 1 cm grating with no limit in sight. Further, regenerating under a temperature profile allows complex spectral shaping of the grating profile. As an example, we have reported broadband chirping by more than Δλ < 9 nm over 1 cm using this approach. High temperature viscoelastic tuning therefore offers a simple and powerful low cost way of tuning the properties of fibre Bragg gratings using a single phase mask.
We present an optical fiber bend sensor with enhanced resolution based on the principle of a Mach-Zehnder interferometer in transmission. The sensor is based on two identical Long-Period Gratings separated by approximately 100 mm in a D-shaped single-mode optical fiber. The sensor provides a narrow resonance bandwidth compared to a typical resonance from a Long-Period Grating. The sensor was recoated with low refractive index polyimide and embedded on a fiber-glass base plate before it was characterized as a bending sensor.
A novel temperature compensation method for long-period grating (LPG) based biochemical sensor is proposed and
experimentally demonstrated. Special laser-heated fiber (LHF) is used for fabricating LPG sensor whose temperature
cross-sensitivity can be compensated by laser heating. Experimental results show that such a method is very promising
for biochemical sensing and can be used to compensate temperature variation of ~86°C.
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