Surface plasmon resonance sensor interrogation with cladding modes excited by tilted fiber Bragg grating

The objective of this Thesis is to develop novel schemes in surface Plasmon resonance (SPR) sensing with Bragg gratings. The thesis focuses on research studies on the sensing characteristics of four novel configurations of SPR and surrounding refractive indices (SRI) sensors.;Firstly, a novel SRI measurement scheme has been demonstrated using a double-clad fiber coupler (DCFC) and a fiber Bragg grating (FBG) to capture cladding modes in reflection. The optical spectra and power in the cladding modes are obtained through the use of a specially designed DCFC spliced to a highly reflective FBG written into slightly cladding-etched standard photosensitive single mode fiber to match the diameter to the inner cladding of the DCFC. The device is capable of capturing low and high order backward propagating cladding modes simply and efficiently. The device is capable of measuring the SRI with an extremely high sensitivity with a total power drop by ≈91% of its initial value and a resolution of 1.433x10-5 RIU between 1.37 and 1.45 RIU. The device provides a large SRI operating range from 1.30 to 1.45 RIU with sufficient discrimination for all individual captured cladding modes. The proposed scheme can be adapted to many different types of bend, temperature, refractive index and other evanescent wave based sensors.;Secondly, a novel optical fiber SPR sensor scheme is demonstrated using reflected guided cladding modes captured by the above mentioned DCFC, and excited in a gold-coated fiber with a tilted Bragg grating (TFBG). This new interrogation approach which is based on the reflection spectrum provides an improvement in the operating range of the device over previous techniques. The device allows detection of SPR in the reflected spectrum and also in the transmitted spectrum as well, making it far easier in comparison with previous standard techniques which use the transmission spectrum. The sensor has a large operating range from 1.335 to 1.432 RIU, and a sensitivity of 510.5 nm/RIU. The device shows strong dependence on the polarization state of the guided core mode which can be used to turn the SPR on or off.;The third scheme is an in-line, multichannel SPR sensor scheme excited with a TFBG inside a chromium and gold-coated fiber. The channels are imprinted in one single optical fiber and each has a different operating wavelength, different TFBG tilt angle, and hence different refractive index operating range. This system, as a multiplexer in a single optical fiber for a number of TFBG-SPR sensors, is demonstrated for the first time. The polarization state of each channel based on the TFBG orientation can be used to switch each SPR sensor on or off as required. This scheme provides an operating range from 1.40 to 1.44 RIU, and a sensitivity of around 500 nm/RIU.;The fourth scheme is a silicon-coated multi-channel SPR sensor scheme excited with a TFBG where different types of fibers have been used and each represents a single separate channel. The high-index dielectric layer silicon coating is demonstrated and characterized for the first time in such am SPR-TFBG sensor. Each channel shows a unique refractive index operating range. The silicon layer is used to tune the SPR refractive index operating range to lower values. This system provides a refractive index operating range from 1.30 to 1.435 RIU, which to our knowledge, is the largest reported so far of such SPR-TFBG sensors. Adding the silicon layer improves the sensor sensitivity to ∼700 nm/RIU and shows stronger polarization dependence. The flexibility and the improved ease of use of the proposed scheme can be adapted to many SPR-TFBG based sensors applications.;The FBGs and the TFBGs proposed in this project have been written by different techniques. A new high-repetition rate 224 nm laser source was used for the FBG fabrication using a scanning phase mask technique with a maximum average writing power of 280 mW at 224 nm. The TFBGs were written using a commercially available 213 nm Q-switched nanosecond Nd:VO4 laser and a newly developed scanned phase-mask-based Talbot interferometer. The 213 nm laser produces pulses of 7 ns with an energy of 10 microJ at a repetition frequency between 0.1 and 30 kHz with a maximum average power of 130 mW. By using the interferometer, the Bragg wavelength can be chosen independently of the UV laser wavelength by changing the mutual angle between the mirrors of the interferometer and the fiber plane. To fabricate tilted gratings with interferometer, the fiber holder is rotated relative to the propagation axis of the fiber.;The proposed configurations aim to improve the performance of SPR biosensors towards miniaturization, better discrimination, increased sensitivity, and multi-analyte sensing which can add significant improvement to existing surface Plasmon resonance sensors using tilted fiber Bragg gratings.