Optical sensor based on hybrid LPG/FBG in D-fiber for simultaneous refractive index and temperature measurement

A dual-parameter optical sensor has been realized by UV-writing a long-period and a Bragg grating structure in D-fiber. The hybrid configuration permits the detection of the temperature from the latter and measuring the external refractive index from the former responses, respectively. The employment of the D-fiber allows as effective modification and enhancement of the device sensitivity by cladding etching. The grating sensor has been used to measure the concentrations of aqueous sugar solutions, demonstrating the potential capability to detect concentration changes as small as 0.01%.

Simultaneous measurement of refractive index and temperature using multimode microfiber-based dual Mach-Zehnder interferometer

A multimode microfiber (MMMF)-based dual Mach-Zehnder interferometer (MZI) is proposed and demonstrated for simultaneous measurement of refractive index (RI) and temperature. By inserting a section of MMMFsupporting a few modes in the sensing arm of the MZI setup, an inline interference between the fundamental mode and the high-order mode of MMMF, as well as the interference between the high-order mode of MMMF and the reference arm, i.e., the dual MZI, is realized. Due to different interference mechanisms, the former interferometer achieves RI sensitivity of 2576.584 nm/RIU and temperature sensitivity of 0.193 nm/°C, while the latter one achieves RI sensitivity of 1001.864 nm/RIU and temperature sensitivity of 0.239 nm/°C, demonstrating the ability to attain highly accurate multiparameter measurements. © 2014 Optical Society of America.

A distributed and key-position fiber sensing system based on LPFG and WFBG assisted OTDR

An enhanced fiber sensing system used for distributed bending and key-position sensing is reported by integrating WFBGs, LPFG and OTDR, which also achieves strain and temperature sensitivities up to 0.047mv/με and 0.675mv/°C respectively. © 2014 OSA.

Refractive index and temperature sensitivity characteristics of a micro-slot fiber Bragg grating

Fabrication and characterization of a UVinscribed fiber Bragg grating (FBG) with a micro-slot liquid core is presented. Femtosecond (fs) laser patterning/chemical etching technique was employed to engrave a micro-slot with dimensions of 5.74 μm(h) × 125 μm(w) × 1388.72 μm(l) across the whole grating. The device has been evaluated for refractive index (RI) and temperature sensitivities and exhibited distinctive thermal response and RI sensitivity beyond the detection limit of reported fiber gratings. This structure has not just been RI sensitive, but also maintained the robustness comparing with the bare core FBGs and long-period gratings with the partial cladding etched off. © 2012 Optical Society of America.

Viscosity of aged bio-oils from fast pyrolysis of beech wood and miscanthus:shear rate and temperature dependence

The viscosity of four aged bio-oil samples was measured experimentally at various shear rates and temperatures using a rotational viscometer. The experimental bio-oils were derived from fast pyrolysis of beech wood at 450, 500, and 550 °C and Miscanthus at 500 °C (in this work, they were named as BW1, BW2, BW3, and MXG) in a bubbling fluidized bed reactor. The viscosity of all bio-oils was kept constant at various shear rates at the same temperature, which indicated that they were Newtonian fluids. The viscosity of bio-oils was strongly dependent upon the temperature, and with the increase of the temperature from 30 to 80 °C, the viscosity of BW1, BW2, BW3, and MXG decreased by 90.7, 93.3, 92.6, and 90.2%, respectively. The Arrhenius viscosity model, which has been commonly used to represent the temperature dependence of the viscosity of many fluids, did not fit the viscosity-temperature experimental data of all bio-oils very well, especially in the low- and high-temperature regions. For comparison, the Williams-Landel-Ferry (WLF) model was also used. The results showed that the WLF model gave a very good description of the viscosity-temperature relationship of each bio-oil with very small residuals and the BW3 bio-oil had the strongest viscosity-temperature dependence.

The accuracy of parameter estimation from noisy data, with application to resonance peak estimation in distributed Brillouin sensing

Distributed Brillouin sensing of strain and temperature works by making spatially resolved measurements of the position of the measurand-dependent extremum of the resonance curve associated with the scattering process in the weakly nonlinear regime. Typically, measurements of backscattered Stokes intensity (the dependent variable) are made at a number of predetermined fixed frequencies covering the design measurand range of the apparatus and combined to yield an estimate of the position of the extremum. The measurand can then be found because its relationship to the position of the extremum is assumed known. We present analytical expressions relating the relative error in the extremum position to experimental errors in the dependent variable. This is done for two cases: (i) a simple non-parametric estimate of the mean based on moments and (ii) the case in which a least squares technique is used to fit a Lorentzian to the data. The question of statistical bias in the estimates is discussed and in the second case we go further and present for the first time a general method by which the probability density function (PDF) of errors in the fitted parameters can be obtained in closed form in terms of the PDFs of the errors in the noisy data.

The accuracy of parameter estimation from noisy data, with application to resonance peak estimation in distributed Brillouin sensing

Distributed Brillouin sensing of strain and temperature works by making spatially resolved measurements of the position of the measurand-dependent extremum of the resonance curve associated with the scattering process in the weakly nonlinear regime. Typically, measurements of backscattered Stokes intensity (the dependent variable) are made at a number of predetermined fixed frequencies covering the design measurand range of the apparatus and combined to yield an estimate of the position of the extremum. The measurand can then be found because its relationship to the position of the extremum is assumed known. We present analytical expressions relating the relative error in the extremum position to experimental errors in the dependent variable. This is done for two cases: (i) a simple non-parametric estimate of the mean based on moments and (ii) the case in which a least squares technique is used to fit a Lorentzian to the data. The question of statistical bias in the estimates is discussed and in the second case we go further and present for the first time a general method by which the probability density function (PDF) of errors in the fitted parameters can be obtained in closed form in terms of the PDFs of the errors in the noisy data.

800 nm WDM interrogation system for strain, temperature, and refractive index sensing based on tilted fiber Bragg grating

A low-cost high-resolution wavelength-division-multiplexing (WDM) interrogation system operating around 800 nm region with operational bandwidth up to 60 nm and resolution of 12.7 pm utilizing a tilted fiber Bragg grating (TFBG) and a CCDarray detector has been implemented. The system has been evaluated for interrogating fiber Bragg grating based strain, temperature sensors, giving sensitivities of 0.59 pm/µe and 5.6 pm/°C, which are in good agreement with previously reported values. Furthermore, the system has been utilized to detect the refractive index change of sample liquids, demonstrating a capability of measuring index change as small as 10¯5. In addition, the vectorial expression of phase match condition and fabrication of TFBG have been discussed.

The application of fibre Bragg grating networks as strain sensors and as phased array antenna true time delay elements

The fabrication of in-fibre Bragg gratings, and the application of arrays of such gratings as strain sensors and as true time delay elements for the control of phased array antennas is reported. Chirped period Bragg gratings were produced using the fibre deformation fabrication technique, with chirps of between 2.9nm and 17.3nm achieved. Arrays of 5mm and 2mm long uniform period Bragg gratings were fabricated using the inscription method, for use as true time delay elements,dissimilar wavefronts and their spectral characteristics recorded. The uniform period grating arrays were used to create minimum time delays of 9.09ps, 19.02ps and 31ps; making them suitable for controlling phased array antennas operating at RF frequencies of up to 3GHz, with 10° phase resolution. Four 4mm long chirped gratings were produced using the dissimilar wavefronts fabrication method, having chirps of 7nm, 12nm, 20nm and 30nm, and were used to create time delays of between 0.3ps and 59ps. Hence they are suitable for controlling phased array antennas at RF frequencies of up to 48GHz. The application of in fibre Bragg gratings as strain sensors within smart structure materials was investigated, with their sensitivity to applied strain and compression measured for both embedded and surface mounted uniform period and fibre Fabry-Perot filter gratings. A fibre Bragg grating sensor demultiplexing scheme based on a liquid crystal filled Fabry-Perot etalon tuneable transmission filter was proposed, successfully constructed and fully characterised. Three characteristics of the LCFP etalon were found to pose operational limitations to its application in a Bragg grating sensor system; most significantly, the resonance peak wavelength was highly (-2,77nm/°C) temperature dependent. Several methods for minimising this temperature sensitivity were investigated, but enjoyed only limited success. It was therefore concluded that this type (E7 filled) of LCFP etalon is unsuitable for use as a Bragg grating sensor demultiplexing element.

Tilted fibre Bragg grating based 800nm WDM interrogation system for strain, temperature and refractive index sensing

We report the implementation of a low-cost high-resolution WDM interrogation system operating around 800nm region with bandwidth up to 60 nm and resolution of 13 pm by utilising a tilted fibre Bragg grating as an out-coupling device and a CCD-array detector. The system has been evaluated for interrogating fibre Bragg grating based strain, temperature sensors, giving sensitivities of 0.6 pm/µe and 5.6 pm/°C which are in good agreement with previously reported values. Furthermore, the system has been utilised to detect refractive index change of sample liquids, demonstrating a capability of measuring index change as small as 10-5.

Tilted fibre Bragg grating based 800nm WDM interrogation system for strain, temperature and refractive index sensing

We report the implementation of a low-cost high-resolution WDM interrogation system operating around 800nm region with bandwidth up to 60 nm and resolution of 13 pm by utilising a tilted fibre Bragg grating as an out-coupling device and a CCD-array detector. The system has been evaluated for interrogating fibre Bragg grating based strain, temperature sensors, giving sensitivities of 0.6 pm/µe and 5.6 pm/°C which are in good agreement with previously reported values. Furthermore, the system has been utilised to detect refractive index change of sample liquids, demonstrating a capability of measuring index change as small as 10-5.

Optic sensors of high refractive-index responsivity and low thermal cross sensitivity that use fiber Bragg gratings of >80° tilted structures

For the first time to the authors' knowledge, fiber Bragg gratings (FBGs) with >80° tilted structures have been fabricated and characterized. Their performance in sensing temperature, strain, and the surrounding medium's refractive index was investigated. In comparison with normal FBGs and long-period gratings (LPGs), >80° tilted FBGs exhibit significantly higher refractive-index responsivity and lower thermal cross sensitivity. When the grating sensor was used to detect changes in refractive index, a responsivity as high as 340nm/refractive-index unit near an index of 1.33 was demonstrated, which is three times higher than that of conventional LPGs.

Optic sensors of high refractive-index responsivity and low thermal cross sensitivity that use fiber Bragg gratings of >80° tilted structures

For the first time to the authors' knowledge, fiber Bragg gratings (FBGs) with >80° tilted structures nave been fabricated and characterized. Their performance in sensing temperature, strain, and the surrounding medium's refractive index was investigated. In comparison with normal FBGs and long-period gratings (LPGs), >80° tilted FBGs exhibit significantly higher refractive-index responsivity and lower thermal cross sensitivity. When the grating sensor was used to detect changes in refractive index, a responsivity as high as 340 nm/refractive-index unit near an index of 1.33 was demonstrated, which is three times higher than that of conventional LPGs. © 2006 Optical Society of America.

In-fibre Bragg grating temperature sensor system for medical applications

A novel quasidistributed in-fiber Bragg grating (FBG) temperature sensor system has been developed for temperature proving in vivo in the human body for medical applications, e.g., hyperthermia treatment. This paper provides the operating principle of FBG temperature sensors and then the design of the sensor head. High-resolution detection of the wavelength-shifts induced by temperature changes are achieved using drift-compensated interferometric detection while the return signals from the FBG sensor array are demultiplexed with a simple monochromator which offers crosstalk-free wavelength-division-multiplexing (WDM). A “strain-free” probe is designed by enclosing the FBG sensor array in a protection sleeve. A four FBG sensor system is demonstrated and the experimental results are in good agreement with those obtained by traditional electrical thermocouple sensors. A resolution of 0.1°C and an accuracy of ±0.2°C over a temperature range of 30-60°C have been achieved, which meet established medical requirements.

Photonic skin for pressure and strain sensing

In this paper, we report on the strain and pressure testing of highly flexible skins embedded with Bragg grating sensors recorded in either silica or polymer optical fibre. The photonic skins, with a size of 10cm x 10cm and thickness of 1mm, were fabricated by embedding the polymer fibre or silica fibre containing Bragg gratings in Sylgard 184 from Dow Corning. Pressure sensing was studied using a cylindrical metal post placed on an array of points across the skin. The polymer fibre grating exhibits approximately 10 times the pressure sensitivity of the silica fibre and responds to the post even when it is placed a few centimetres away from the sensing fibre. Although the intrinsic strain sensitivities of gratings in the two fibre types are very similar, when embedded in the skin the polymer grating displayed a strain sensitivity approximately 45 times greater than the silica device, which also suffered from considerable hysteresis. The polymer grating displayed a near linear response over wavelength shifts of 9nm for 1% strain. The difference in behaviour we attribute to the much greater Young's modulus of the silica fibre (70 GPa) compared to the polymer fibre (3 GPa).