Characteristics of cladding modes in a photonic crystal fiber (PCF) with triangular air-hole lattice in the cladding are numerically analyzed using a finite element method. The transition for LP11 cladding mode to core mode with variation of the normalized wavelength has been shown. The transition of the LP01 cladding mode to the outer silica mode and reorganization of the LP0m cladding modes caused by varying the fiber radius has been investigated. By choosing the optimized fiber radius, which is located in the cladding modes’ reorganization region, the sensitivity of the coupled wavelength between the core mode LP01 and cladding mode LP03 to surrounding refractive index is increased by a factor of five and reaches to 2660 nm/refractive index unit over the range of 1.40 to 1.42. The sensitivity is competitive with that of long-period grating in PCF in response to changes in refractive indices of the medium contained in the cladding air channels.
The mode transition of high-order cladding modes in a coated long-period fiber grating (LPFG) was studied using coupled-mode theory, the response characteristic of cladding mode effective index was analyzed with increasing overlay thickness. It was found through analyses that the shift of resonant wavelength in the mode transition region was larger than that in the non-mode transition region. Further more, the phase-matching curves for 19th cladding mode was investigated when the overlay thickness was located in the mode transition and non-mode transition regions, the shift between two resonance wavelengths of dual peak in the mode transition region was bigger than that in non-transition mode region. The response characteristics of film refractive index of coated LPFG was investigated for a high-order cladding mode while the overlay thickness was located in mode transition and non-transition mode regions, the positions and amplitudes of the dual peak of the high-order cladding mode had different changes when the film refractive index vary slightly. The sensor sensitivity was defined and calculated. The result shows that the sensitivity of film refractive index could reach 69.12 and 0 respectively, when the overlay thickness was located in mode transition and non-mode transition regions. The corresponding resolutions of film refractive index can be available to 10-7 in mode transition region.
Band-stop optical fiber filters based on linear chirped long-period fiber gratings (LPFGs) were investigated. The transmission characteristics of the linear chirped LPFGs were analyzed in detail with transfer matrix method. Linear chirp coefficient had a remarkable influence on the bandwidth of the transmission spectra. When the linear chirp coefficient was 10 −9 and the transmission spectra corresponding to multiple cladding modes were superposed, the bandwidth of the transmission spectra could be expanded to >1000 nm . So the linear chirped LPFGs can be used for single-channel broadband band-stop filters, and the relationships between the grating parameters and the bandwidth of the transmission spectra were given. When the linear chirp coefficient is 10 −8 , several narrow and separated loss peaks appeared in the transmission spectra, which indicated that the linear chirped LPFGs can be used for multichannel narrowband filters. The variations of the grating parameters caused the shifts of the transmission spectra and the changes of the intervals between separated loss peaks. The conclusions that are significant in optimizing the fiber grating filters’ design were obtained.
A cascaded long-period fiber grating (CLPFG) sensor with film coating is presented. Two long-period fiber gratings (LPFGs) are cascaded to form a Mach-Zehnder interferometer. The optical transmission spectrum and the sensing characteristics for ambient refractive index (RI) measurement of the CLPFG sensor are analyzed in a form of transfer matrix based on rigorous coupled-mode theory. The results indicate that it is highly sensitive to the film RI and surrounding RI, so it can be used as a gas sensor or a solution sensor. Data simulation shows that the resolution of the RI of the films is predicted to be 10 −7 . Further, a cascaded chirped LPFG is introduced to study its sensing performance. The influence of the chirp coefficient and the grating structural parameters on the transmission spectrum of the cascaded chirped LPFG is analyzed. Data simulation shows that the resolution of the RI of the films is predicted to be 10 −9 . In contrast to the conventional measurement method based on interrogating the wavelength change, intensity detection of this cascaded chirped LPFG sensor means that no optical spectrum analyzer is required in the measuring system, which is favorable for practical applications, especially for in situ environmental motoring.
A cascaded long-period fiber grating (CLPFG) sensor with film coating is presented in this paper. Two LPFGs are
cascaded to form a Mach-Zehnder interferometer. The optical transmission spectrum and the sensing characteristics for
ambient refractive index measurement of the cascaded LPFG sensor are analyzed in a form of transfer matrix based on
rigorous coupled mode theory. The results indicate that it is highly sensitive to the film refractive index and surrounding
refractive index, so it can be used as gas sensor or solution sensor. In addition, the influence of the film optical
parameters on the sensitivity is analyzed. By using optimization method, the optimal film optical parameters and the
grating structure parameters are obtained. Data simulation shows that the resolution of the refractive index of the films is
predicted to be 10-7. Further, the cascaded chirped LPFG is introduced to study its sensing performance. The influence of the chirp coefficient and the grating structural parameters on the transmission spectrum of cascaded chirped LPFG is analyzed. Data simulation shows that the resolution of the refractive index of the films is predicted to be 10-9. In contrast to conventional measurement method based on interrogating the wavelength change, the intensity detection of this cascaded chirped LPFG sensor means that no optical spectrum analyzer is required in the measuring system, which is favorable for practical applications, especially for in situ environmental motoring.
A novel metal coated long period fiber grating (LPFG) liquid sensor is presented, in which the dual-peak resonance (DPR) combined with the surface plasma resonance is utilized to improve the sensitivity to liquid concentration. Based on the coupled-mode theory, the double-clad structural model is used to analyze the dual-peak resonant characteristics and refractive index sensing properties of this LPFG. The results show that dual resonant peaks are far away from each other, and shift in opposite directions with an increase of surrounding refractive index. In addition, the influence of metal film thickness and grating structural parameters (grating period Λ, average index change σ) on the sensitivity to refractive index is discussed for this LPFG sensor, which provides a foundation for designing high sensitivity liquid LPFG sensors. Experimentally, the silver films were coated on a LPFG by direct current sputtering, and the dual resonant peaks were observed in this LPFG. Then the salt solution concentration monitoring test was performed. The results indicate the metal coated LPFG based on DPR has higher refractive index sensitivity as compared with dual-peak based non-coated LPFG and general single peak based LPFG, the resolution of liquid refractive index is available to 10−5 for the proposed LPFG with suitable film and grating parameters.
A dual-peak resonance in coated long-period fiber grating (LPFG) is presented in this paper. By resolving the
characteristic equation of the coated LPFG, the dual peak resonant wavelengths are determined based on rigorous
coupled mode theory. The relationships between the dual peak resonant wavelengths and grating period and the mode
ordinal are studied. The results show that the dual resonance appears in higher cladding mode, and the higher the
cladding mode ordinal is, the smaller the related grating period required to couple with core mode is. Further, the
influence of film optical parameters (the film thickness h3 and the refractive index n3) on the intervals of dual resonant
wavelengths, as well as the attenuation peak of transmission spectra, are analyzed whether the optical dispersion is under
consideration or not. The transmittance spectra show the dual resonant peaks shift in the opposite direction with the
variation of the film refractive index. This can be used to construct a refractive index sensor, in which the films sensitive
to the surrounding gases are coated on the cladding of the fiber grating region, and the intervals of dual peak resonant
wavelengths change with the film refractive index. By using optimization method, the optimal film optical parameters
and the grating structure parameters are obtained. Data simulation shows that the resolution of the refractive index of the
films is predicted to be more than 10-7. The theoretic analysis provides straightforward foundation for the actual highly
sensitive film sensors.
Based on rigorous coupled mode theory, a theoretical model is established for studying the optical characteristics of
long-period fiber grating (LPFG) coated with the sensing thin films. The vector components of the electric field and the
local intensity curves for the lowest order cladding mode are plotted to study the field distribution of cladding mode. It is
found that the transverse field components of HE11 cladding mode are approximately 102 times larger about than the
longitudinal field components, and the low order HE modes have a larger proportion of intensity localized in the core
than the low order EH modes, just like the double-clad LPFG. Further, the influences of the sensing film optical
parameters and grating structure parameters on the attenuation peak of transmission spectra are analyzed. Data
simulation shows that the sensitivity to the refractive index of sensing films is predicted to be more than 10-7. The
optimal design parameters of the LPFG film sensor for higher sensitivity are ascertained by plotting the contour of the
sensor sensitivity. Experimentally, the sol-gel derived SnO2 film LPFG was prepared, and a preliminary gas-sensing test
for detection of C2H5OH was performed. The results indicate that the LPFG film sensor with structure optimization has
higher sensitivity, and the detection sensitivity is available to 10-1ppm on the condition of optimum optical parameters.
With the advantages of both film sensors and fiber sensors, the coated LPFG sensor has a wide and promising application
prospect in process analytical chemistry, environmental monitoring, and biochemical sensing.
A new scheme of optical multichannel analyzer is developed based on Model 4400 Signal Detection and Analysis System. The scheme consists of an optical signal source, a photodiode array and Model 4400 Signal Detection and Analysis System. The signal source provides varies light intensity distribution from optical system. The photodiode array transforms spatial distribution of light intensity into time dependence of electric current intensity. Model 4400 Signal Detection and Analysis System carries out signal processing and analysis through boxcar, keyboard control and advanced microprocessor. With this scheme, the spatial distribution of laser light-intensity was described and the relevant parameters such as beam waist radius of Gaussian distribution were obtained. Further, the optical parameters of the plane glass surface and PMTES thin films were obtained by p-polarized reflectance method. By measuring the angle distribution of reflectance ratio and fitting the results with theoretical data, the optical parameters can be obtained easily. Experimental results indicate that the refractive index and extinction coefficient of glass surface layers decrease exponentially with the thickness of glass layer. Also the optical parameters of PMTES films with the presented scheme have been measured, and the experimental results coincide well with theoretical simulation only if the glass surface layers are considered. Due to the introduction of digital averaging technique in 4400 System, the sampling signals with higher signal-to-noise ratio are acquired, and the presented scheme has higher measurement precision, which is very suitable to temporal-spatial transform and analysis of optical field and precise measurement of surface and film system.
Sb-doped SnO2/SiO2 nano-composite thin films prepared by sol-gel dip-coating method have been studied. By using X-ray diffraction (XRD), atomic force microscopy (AFM) and Fourier transform infrared (FTIR) spectroscopy, detailed investigation on the structure and morphology of the films has shown the crystalline grain size of Sb-doped SnO2/SiO2 thin films is about 34nm, with larger specific surface area and duty porosity, which is fit for gas-sensing materials. The adulteration of SiO2 particles leads to the condensation of Sn-OH and the strengthening of gel network, and improve the adhesion of the films. In addition, the optical properties of the thin films were studied by UV-Vis spectra and p-polarized light reflectance angular spectrum. The results showthat the optical transmissivity of Sb-doped SnO2/SiO2 thin films is higher, near 95% in visible spectrum range, the measured optical gap is found equal to 3.67eV, also the films take on smaller refractive index and extinction coefficient compared with those of the SnO2 and Sb:SnO2 films, which is compatible with the semiconductor substrate in the solar cell. Further, the gas-sensing test was made to three kinds of gas C3H8, C2H5OH and NH3 in our novel high sensitive scheme for optical film sensors. The results indicate that Sb doping to SnO2 films greatly improves the gas sensitivity to C2H5OH, and the gas sensitivity of Sb:SnO2/SiO2 nano-composite thin films are higher than that of Sb:SnO2 thin films. The detection sensitivity of this optical film sensor is available to 10-1ppm provided that the resolution of reflectance ratio is 10-2.
The response of a long-period grating coated with the sol-gel derived films on the cladding of the fiber grating region to surrounding medium (gases) was studied in this paper based on coupled-mode theory. A four-layered numerical model is also developed to determine the dependence of the central wavelengths of the attenuation bands on the thin film optical parameters ( thickness d3 and refractive index n3). By analyzing the relation between the sensitivity Sn and the thin film optical parameters and the fiber grating parameters (the grating period, the core index change and the grating length ), the optimal optical parameters of thin film layer of the sensor is obtained. Data simulation shows that the sensitivity of this scheme to refractive index of the films is predicted to be more than 107. In the end a gas-sensitive long-period grating sensor is fabricated according to the theoretical optimization results. The preliminary gas-sensing experiment was performed, and a novel LPFG gas sensor special to C2H5OH and CH4 was established.
A new method is proposed for measurement of the film optical parameters. A p-polarized laser beam falls on the glass surface at θi, and two reflected beams with intensities Ia and Ib from the front and the rear surface of the glass are
detected. The characteristic of the angular modulation of reflectance ratio γ ( γ =Ia/Ib) is closely related to the film optical parameters, namely refractive index nf, extinction coefficient kf and thickness df. By means of measuring the angle distribution γ(θi) and fitting the results with theoretical data, the optical parameters of the thin films can be obtained easily. Experimentally, we measured the exact optical parameters of tetra-neopentoxy phthalocyanine Zinc(TNPPcZn) LB films. In addition, the changes of optical parameters and absorption curves of TNPPcZn LB films in visible range at different annealing temperature were investigated. The results indicate that the changing tendency of the extinction
coefficient of TNPPcZn LB films obtained from these two methods were coincident. When the annealing temperature increased to 150°C, the monomer of TNPPcZn in LB films transformed to aggregate, nf and kf of the films increased. Further, nf and kf decreased as aggregate changed back to monomer again at 300°C.
The existence of glass surface layers has a great influence upon the optical properties of the thin films coated on glass substrate, particularly upon laser-induced damage threshold. A new and simple method is proposed to analyze the glass surface layers based on p-polarized light reflectance. Experimentally, the glasses with different cleaning treatments have been analyzed. The results show that the glass with acid erosion has smaller extinction coefficient, and the cleanliness and the laser damage threshold increase accordingly. Furthermore, we measure the laser-induced damage thresholds of SiO2 antireflective films coated on several glass substrates above-mentioned. It is indicated that the laser-induced damage threshold under the acid and alkaline cleaning treatment increase by about 80% over the water cleaning treatment.
The sol-gel derived metal oxide semiconductor films have been used as sensing medium in the opto-chemical sensor based on SPR for the first time. Data simulation shows that the sensitivity of this scheme to refractive index of the films is predicted to be more than 105. By adding an intermediate layer between the metal films and the sensing films, optimizing the optical parameters, the sensitivity of this sensor with optimal optical parameters can increase by 40% over that of only sensing films. Experimentally, we have selected SnO2 and SiO2 films as sensing and intermediate layers, and made the gas sensing tests to NH3, C2H5OH and C3H8. The sensor with optimized structure has a higher sensitivity, and the detection limits for these gases are available to 10-1 ppm.
A new scheme of sensor is presented in this paper. The optimization of parameters of the sensor is carried out for designing optical sensor with high sensitivity. By data simulation, the resolutions of nf, kf and df of the film sensor are predicted to be less than 10-7, 10-5, 10-3 nm, respectively. The optical parameters of sol-gel SnO2 film were measured under different dip rates and annealing temperatures, and the sensitivity to the gas C3H8 was investigated. The results indicate that the detection limit is available to 10-1 ppm on the condition of optimum optical parameter and incident angle.
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