Long-Period Fiber Grating Probe: An Improved Design Suitable for Biosensing

Add-layer sensitivity of long-period fiber grating (LPFG) near mode transition (MT) has been studied in reflection configuration to realize a highly sensitive LPFG-based sensor probe suitable for biosensing. The dependency of the sensitivity on the separation between the resonant cladding modes considered during MT has also been investigated. The separation between two consecutive cladding modes was precisely increased or stretched to a desired value by reducing the cladding diameter. The probe was formed by cleaving the mode stretched LPFG (MSLPFG) through the grating region. The MSLPFG was designed to operate near MT by deposition of electrostatic self-assembled (ESA) polymeric overlay layers on the grating surface. We demonstrate that the polymeric overlay not only increases the sensitivity of the sensor but also eliminates undesirable multiple resonant bands that appear due to the introduction of arbitrary phase while cleaving the grating. Add-layer sensitivity of the MSLPFG sensor probe was obtained and found to be ~2.0 nmWL/nmTH with a significantly enhanced peak attenuation of
$\sim -30$
dB around the MT region.