Investigation Of Optical Sensing Performance Of Integrated Rare-earth Doped Microsphere Resonator Devices

Due to their unique photoluminescence properties,rare-earth doped glass materials have been explored and developed for a number of traditional applications such as lasers,amplifiers and other important optical research areas.During the last few decades,based on state-of-the-art fabricating techniques,the application of micro-lasers,especially whispering-gallery mode（WGM）micro-resonator lasers,in chemical and biological sensing has increased due to advances made in reducing the gap between laboratory experiments and their real-world applications.Combining the unique properties of rare-earth doped glass with the WGM resonances make it possible to obtain light emission via rare-earth transitions at different wavelength bands.A range of further promising applications in different areas such as biology,molecular spectroscopy and environmental monitoring are feasible so far.Rare-earth doped glass based WGM microspheres can be fabricated easily and have drawn significant attention because of their added advantages compared with WGM resonators,including narrow linewidth,strong confinement,small volume and ease of integration.However,the traditional microsphere devices are usually excited by near-field coupling methods,which results the coupling system always suffers from environmental turbulence due to the poor mechanical stability.Therefore,in this thesis,a novel WGM integrated device based on the rare-earth doped microsphere embedded into suspended core hollow fiber is designed and manufactured,which makes the entire integrated device more compact and stable,compared with tapered fibre coupling method based devices.Besides,the optical properties and sensing applications of this integrated device have been characterized and investigated.Above all,the main research contents of this thesis are as follows:1.Based on the basic principle of tapered fiber-microsphere coupling system,the suspended tri-core hollow fiber（STCHF）was designed and fabricated.Thanks to the special structure of the suspended fiber cores and the air hole of the STCHF,the microsphere was mechanically placed inside the STCHF.Then the transmission spectrum of this integrated device was measured and the WGM of the microsphere was obtained.Compared with other near-field coupling methods using tapered fiber,the performance of the mechanical stability of this novel integrated device was strongly improved.2.The Er³⁺-Yb³⁺co-doped lead-germanate compound glass and glass microspheres are fabricated respectively and the compound glass microspheres were placed into the STCHF.Both the compound glass and the compound glass microspheres have shown strong up-conversion fluorescence emissions using a 980 nm pump laser.Meanwhile,the behaviors of up-conversion fluorescence of glass and glass microsphere can vary with the doping concentration ratio of Er³⁺and Yb³⁺ions and the power of the 980 nm laser.Therefore,such variations of up-conversion fluorescence of glass and glass microsphere have shown a great potential application in the area of colorful imaging.3.Based on the special 4f energy levels,rare-earth activators have been extensively adopted as temperature measurement when they are doped into appropriate hosts.In this chapter,Er³⁺-Yb³⁺co-doped tellurite glass microspheres are fabricated by the powder floating method and the samples are placed into the STCHF.Then the green luminescence emissions at 528 nm and 549 nm of the tellurite glass microsphere are obtained using a 980 nm near infrared laser excitation.And the fluorescence intensity ratio（FIR）between such two wavelengths can vary with surrounding temperature,because the ²H_11/2 and ⁴S_3/2 levels of the Er³⁺ions are thermally coupled and the transition ²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 can be used for optical thermometry.The temperature sensing properties were studied at the temperature range of 300-380 K and the maximum sensitivity is found to be 38.6×10^-4 K^-1.4.Based on the up-conversion luminescence emissions of Er³⁺ions,the Er³⁺-Yb³⁺co-doped tellurite glass microsphere integrated inside STCHF can be used for ammonia concentration in water detection.In this chapter,Er³⁺-Yb³⁺co-doped tellurite glass microspheres are prepared and packaged inside STCHF.Under 980 nm laser pump,the green and red up-conversion visible luminescence emissions of the glass microsphere were achieved.The main absorption peak of phenol red will shift to 560 nm in alkaline environment,which overlaps with the green luminescence emission of Er³⁺ions.Therefore,the ammonia concentration in water can be detected by monitoring the change of fluorescence intensity ratio between green and red luminescence emission.The limit of detection is 0.5 ppm and the response time is 0.2 s.In this thesis,a novel integrated device consists of rare-earth doped compound glass microsphere and STCHF are investigated comprehensively.The photoluminescence properties of the rare-earth doped glass microspheres are presented using near-infrared laser excitation.Color-tunable up-conversion photoluminescence can be achieved by a number of methods,such as the co-doping ratio and the pump power.The overall results suggest that the proposed integrated device could be an exceptional choice for photoluminescence-based temperature sensing device with a good performance as well as an excellent chemical sensor with high sensitivity for the detection of ammonia concentration in water.