All-fiber ellipsometer for nanoscale dielectric coatings

Multiple mode resonance shifts in tilted fiber Bragg gratings(TFBGs)are used to simultaneously measure the thickness and the refractive index of TiO_(2) thin films formed by Atomic Layer Deposition(ALD)on optical fibers.This is achieved by comparing the experimental wavelength shifts of 8 TFBG resonances during the deposition process with simulated shifts from a range of thicknesses(T)and values of the real part of the refractive index(n).The minimization of an error function computed for each(n,T)pair then provides a solution for the thickness and refractive index of the deposited film and,a posteriori,to verify the deposition rate throughout the process from the time evolution of the wavelength shift data.Validations of the results were carried out with a conventional ellipsometer on flat witness samples deposited simultaneously with the fiber and with scanning electron measurements on cut pieces of the fiber itself.The final values obtained by the TFBG(n=2.25,final thickness of 185 nm)were both within 4%of the validation measurements.This approach provides a method to measure the formation of nanoscale dielectric coatings on fibers in situ for applications that require precise thicknesses and refractive indices,such as the optical fiber sensor field.Furthermore,the TFBG can also be used as a process monitor for deposition on other substrates for deposition methods that produce uniform coatings on dissimilar shaped substrates,such as ALD.

In situ plasmonic optical fiber detection of the state of charge of supercapacitors for renewable energy storage

In situ and continuous monitoring of electrochemical activity is key to understanding and evaluating the operation mechanism and efficiency of energy storage devices.However,this task remains challenging.For example,the present methods are not capable of providing the real-time information about the state of charge(SOC)of the energy storage devices while in operation.To address this,a novel approach based on an electrochemical surface plasmon resonance(SPR)optical fiber sensor is proposed here.This approach offers the capability of in situ comprehensive monitoring of the electrochemical activity(the electrode potential and the SOC)of supercapacitors(used as an example).The sensor adopted is a tilted fiber Bragg grating imprinted in a commercial single-mode fiber and coated with a nanoscale gold film for high-efficiency SPR excitation.Unlike conventional“bulk”detection methods for electrode activity,our approach targets the“localized”(sub-μm-scale)charge state of the ions adjacent to the electrode interface of supercapacitors by monitoring the properties of the SPR wave on the fiber sensor surface located adjacent to the electrode.A stable and reproducible correlation between the real-time charge–discharge cycles of the supercapacitors and the optical transmission of the optical fiber has been found.Moreover,the method proposed is inherently immune to temperature cross-talk because of the presence of environmentally insensitive reference features in the optical transmission spectrum of the devices.Finally,this particular application is ideally suited to the fundamental qualities of optical fiber sensors,such as their compact size,flexible shape,and remote operation capability,thereby opening the way for other opportunities for electrochemical monitoring in various hard-to-reach spaces and remote environments.

Operando optical fiber monitoring of nanoscale and fast temperature changes during photo-electrocatalytic reactions

In situ and continuous monitoring of thermal effects is essential for understanding photo-induced catalytic processes at catalyst's surfaces.However,existing techniques are largely unable to capture the rapidly changing temperatures occurring in sub-μm layers at liquid-solid interfaces exposed to light.To address this,a sensing system based on a gold-coated conventional single-mode optical fiber with a tilted fiber Bragg grating inscribed in the fiber core is proposed and demonstrated.The spectral transmission from these devices is made up of a dense comb of narrowband resonances that can differentiate between localized temperatures rapid changes at the catalyst's surface and those of the environment.By using the gold coating of the fiber as an electrode in an electrochemical reactor and exposing it to light,thermal effects in photo-induced catalysis at the interface can be decoded with a temperature resolution of 0.1℃and a temporal resolution of 0.1 sec,without perturbing the catalytic operation that is measured simultaneously.As a demonstration,stable and reproducible correlations between the light-to-heat conversion and catalytic activities over time were measured for two different catalysis processes(linear and nonlinear).These kinds of sensing applications are ideally suited to the fundamental qualities of optical fiber sensors,such as their compact size,flexible shape,and remote measurement capability,thereby opening the way for various thermal monitoring in hard-to-reach spaces and rapid catalytic reaction processes.