Overwrite fabrication and tuning of long period gratings

The central wavelengths of the resonance bands are critical aspect of the performance of long period gratings (LPGs) as sensors, particularly for devices designed to operate near the phase matching turning point (PMTP), where the sensitivity to measurements can vary rapidly. Generally, LPGs are characterized by their period, but the amplitude of the amplitude of the index modulation is also an important factor in determining the wavelengths of the resonance bands. Variations in fabrication between LPG sensors can increase or decrease the sensitivity of the LPG to strain, temperature or surrounding refractive index. Here, the technique of overwritten UV laser fabrication is demonstrated. It is shown that, on repeated overwriting, the resonance bands of an LPG exhibit significant wavelength shift, which can be monitored and which can be used to tune the resonance bands to the desired wavelengths. This technique is applied to periods in the range 100 to 200 µm, showing the cycle-to-cycle evolution of the resonance bands near the PMTPs of a number of cladding modes. The use of online monitoring is shown to reduce the resonance band sensor-to-sensor central wavelength variation from 10 nm to 3 nm

Properties of length-apodized phase-shifted lpgs operating at the phase matching turning point

The characteristics of length-apodized phase-shifted fiber optic long period gratings with full and partial nanostructured coatings have been explored theoretically and experimentally. The twin rejection bands that are characteristic of length-apodized phase-shifted long period gratings are studied for a long period grating (LPG) operating at the phase matching turning point. When one half of the length of the LPG is coated, complex bandgap like structure appears within the transmission spectrum, which may be of benefit to spectral filter design and for sensing applications

Spectral behavior of thin film coated cascaded tapered long period gratings in multiple configurations

In this work the spectral response of cascaded tapered long period gratings coated by nano-sized polymeric films has been investigated as function of the surrounding medium refractive index (SRI). The investigation was aimed to identify the best configuration in terms of coated/not coated areas in order to fully benefit of the SRI sensitivity enhancement due to the modal transition mechanism of nano-coated long period gratings while preserving the fringes visibility

Fasts laser POF side long period gratings fabrication

By using a fiber laser chirp pulse amplifier femtosecond laser system, long period gratings structures are micro-machined on one side of a Plastic Optical Fiber. Very narrow and high attenuation transmission bands can be obtained.This work was supported by the MCYT (Spain), project CICYT: TEC2010-20224-C02-02 and CONACyT (Mexico) by sabbatical stay at the University of Cantabria, file: 186243

Hydrogen sensing using palladium coated long period gratings

The use of palladium coated optical fibres containing an in-fibre long period grating (LPG) structure for the sensing of low concentrations of hydrogen has been investigated. Previous proof of principle experiments were refined and extended, demonstrating sensitivities of up to 60 pm for 1% hydrogen at an operating temperature of 70°C. Characterisation of the Pd deposition technique (RF sputter coating) including XPS and SEM analysis to investigate surface roughness and contamination were carried out and are discussed. These measurements were prompted by the need to characterise and eliminate sensor drift associated with delamination of the sensor layer. Particular care was taken to characterise the optical properties of Pd thin films, both in the presence of hydrogen and without, through ellipsometry and surface plasmon resonance, since values gained from the literature showed wide variations. The results presented in this thesis differ from published Pd refractive indices but are specific for the conditions used in this work. Techniques to directly measure the refractive index profile within the core of an LPG were investigated and a possible solution is presented. Theoretical models to calculate the transmission spectrum of a LPG, both with and without a Pd layer are presented, discussed and the implications due to the poor characterisation of the LPG are considered. The potential use of higher order double cladding modes (with up to 20 times the sensitivity of the lower mode orders) as a sensing regime is investigated and the practical limitations discussed

Long Period Gratings in Random Hole Optical Fibers for Refractive Index Sensing

We have demonstrated the fabrication of long period gratings in random hole optical fibers. The long period gratings are fabricated by a point-by-point technique using a CO2 laser. The gratings with a periodicity of 450 μm are fabricated and a maximum coupling efficiency of −9.81 dB has been achieved. Sensing of different refractive indices in the surrounding mediums is demonstrated by applying standard liquids with refractive indices from 1.400 to 1.440 to the long period grating

Highly birefringent long period gratings fabricated with femtosecond laser

Long Period Gratings (LPG) in standard fiber have been manufactured with a sharply focused near infrared (NIR) femtosecond laser beam. Polarization splitting of the attenuation bands is strongly dependent upon the inscription power

The use of long-period gratings to detect Sulphur Hexafluoride.

The combination of the increase in cost of Sulphur Hexafluoride (“SF6”), the registration of SF6 as a greenhouse gas and the use of SF6 at pressures above atmospheric has caused the need for SF6 detection equipment to arise. This research report outlines and reviews past and present technologies utilised, together with presently available commercial systems. Along with the review of available technologies, a proposed system utilising opto-electronic sensors to provide real-time detection is discussed. The proposed method investigated is the utilisation of Long-period gratings to detect slight refractive index changes of the ambient environment outside gas insulated systems and attempt to resolve the change in refractive index into detection mechanism for SF6. The method is shown to be possible, however currently not practically feasible. It is recommended that a practical investigation be undertaken to determine the real practical constraints theorised within but the magnitudes not yet established