S. G. Raymond

The effects of oxygen concentration and light intensity on the photostability of zwitterionic chromophores

Photostability measurements at different oxygen partial pressures and light intensities have been made on host-guest films containing amorphous polycarbonate and an organic chromophore with a high second order nonlinear optical figure of merit. We find that the photodegradation quantum efficiency dramatically increases with increasing oxygen partial pressure. At very low oxygen partial pressures (8×10−6 bar) the average number of photons required to photodegrade a chromophore is as high as 2×108 at 655 nm. The photodegradation quantum efficiency in air is observed to decrease with increasing optical intensity. We show that this is due to a reduced oxygen content in the film caused by chromophore photodegradation rather than ground state bleaching. There is an anomalous increase and then decrease in the photoluminescence intensity that cannot easily be explained.

Investigation of polyimide coated fibre Bragg gratings for relative humidity sensing

This paper reports the development of a polyimide coated fibre Bragg grating for relative humidity sensing. The humidity response was measured for several polyimide coatings including P84 and P84 HT. The P84 HT polyimide coating proved the most promising with a large coefficient of moisture expansion measured at 73.8 ppm/%rh. The effect of coating thickness and optical fibre diameter were also investigated.

Development of fibre Bragg grating based strain/temperature sensing system

We have characterised a strain and temperature sensing system being developed by Southern Photonics that uses a new Optical Interrogator and fibre Bragg gratings. We have determined the key strain and temperature coefficients, and shown that strain and temperature can be measured simultaneously. The experimental uncertainty is 5.2 pm when using the 1540 nm fibre Bragg grating, which corresponds to an experimental uncertainty in measuring the temperature of 0.54 ºC and in measuring the strain of 3.4 με. Simulations predict that a Bragg reflection of more than 90% can be achieved for Bragg Gratings in polymer thin films containing chromophores for grating lengths as small as 200 μm. A small Bragg grating length means that it should be possible to create waveguides and four Bragg gratings for strain tensor and temperature measurements within an area as small as 5×5 mm2.

Optics-Based Strain Sensing System

An optics-based strain sensing system is being developed for quazi-distributed strain sensing in locations and environments that are not accessible to conventional strain sensors. The system comprises an Optical Interrogator that has been designed and constructed by Southern Photonics [1], and optical fibre coupled Bragg grating strain sensors. It has been tested using commercial fibre Bragg gratings [2] that were attached to 2 samples of 316 grade stainless steel and cycled in strain and temperature using an Instron mechanical testing machine and temperature controlled cabinet. The results have been compared to the performance of conventional electrical resistance strain gauges. Pairs of fibre Bragg gratings were simultaneously interrogated at 1540 and 1550 nm centre wavelengths to demonstrate the ability of the system to use multiple sensors for quazi-distributed sensing and temperature compensation. The Optical Interrogator resolution is approximately 4 microstrains, accounting for longer term temperature drift, and it is capable of resolving dynamic strains at rates of up to 90 Hz.

Photoluminescence and optical studies of photodegradation in nonlinear optical organic chromophores

Photostability measurements have been made on host-guest films containing amorphous polycarbonate and an organic chromophore with a high 2nd order nonlinear optical figure of merit. We find that the rate of photodegradation strongly depends on the oxygen partial pressure. At very low oxygen partial pressures (1.4×10-5 bar) the average number of photons required to photodegrade a chromophore is as high as 1×109 at 655 nm. Encapsulation leads to an initial rapid decrease in the photodegradation rate due to the trapped oxygen and a gradual photodegradation where 2×109 photons are required to photodegrade a chromophore. There is an anomalous increase and then decrease in the photoluminescence intensity during ultraviolet irradiation.

Refractive index gratings in electro-optic polymer thin films.

Refractive index gratings have been inscribed in polymer thin films by permanently photobleaching the organic chromophore PYR-3 dopant. The grating inscription process was investigated in detail for the purpose of improving the diffraction efficiency (η) of the PYR-3 doped polymer gratings. Three processes were identified that contributed to the η of the first diffracted order: a periodic change in the refractive index due to photobleaching of the PYR-3, formation of the surface relief grating as a consequence of free volume change during bleaching, and the introduction of periodic, strain-induced changes in the refractive index.

PHOTOSTABILITY ENHANCEMENT STUDIES ON ZWITTERIONIC CHROMOPHORES FOR NONLINEAR OPTICS

Optical and photostability measurements have been made on nonlinear optical films containing amorphous polycarbonate and an organic chromophore that has a high 2nd order nonlinear optical figure of merit. We show that the decrease in the photodegradation quantum efficiency with increasing optical intensity can be modelled in terms of oxygen depletion by oxygen-mediated chomophore photodegradation. The addition of a known singlet oxygen quencher, beta carotene, leads to a 1100% decrease in the photodegradation quantum efficiency.

Light losses from scattering in luminescent solar concentrator waveguides

The reductions in the transmission of emission originating from a fluorophore dissolved in a polymer matrix due to light scattering were compared in two forms of planar waveguides used as luminescent solar concentrators: a thin film of poly(methylmethacrylate) (PMMA) spin-coated on a glass plate and a solid PMMA plate of the same dimensions. The losses attributable to light scattering encountered in the waveguide consisting of the thin film of polymer coated on a glass plate were not detectable within experimental uncertainty, whereas the losses in the solid polymer plate were significant. The losses in the solid plate are interpreted as arising from light-scattering centers comprising minute bubbles of vapor/gas, incomplete polymerization or water clusters that are introduced during or after the thermally induced polymerization process.

Photoinitiators for two-photon polymerisation: effect of branching and viscosity on polymerisation thresholds

A series of multi-branched two-photon photoinitiators (PIs) based around the well-known triphenylamine donor core were synthesised for use in two-photon polymerisation (TPP) and are designated as compounds 6, 7 and 8. The use of a phenylene-vinylene π-system was used with an ethyl ester acceptor moiety which gives dipolar (6), quadrupolar (7) and octupolar (8) branching. Two-photon absorption cross-sections (δ2PA) of 126 GM, 358 GM and 590 GM were measured at 780 nm for 6, 7 and 8, respectively. The fluorescence quantum yields (ϕF) were determined in both MeOH and the acrylate system employed for TPP, and demonstrate the impact of viscosity upon photophysical properties of multi-branched molecules. Excellent polymerisation thresholds were demonstrated in the μW region: namely 45 μW (6), 61 μW (7) and 27 μW (8) at a writing speed of 50 μm s−1. Finally, an explanation for the disparity of polymerisation thresholds is proposed for these PIs and provides insight into the future development of low threshold PIs for TPP.

Photoinduced properties of Bis-azo chromophore host guest systems-birefringence and all optical tuneable polymer waveguide Bragg gratings

The photoinduced properties of recently synthesised and reported bis-azo dyes doped host-guest polymer films were investigated. The photochromic properties and photoinduced birefringence were characterised. The birefringence was found to be very stable, and 96% of the birefringence was retained after illumination of the films was stopped. The high two photon absorption cross section (1251GM) together with stable photoinduced birefringence makes these materials promising candidates for 3D optical storage. The reported photochromic properties were subsequently used to design and manufacture an all optical integrated device. A waveguide Bragg grating was fabricated, and the Bragg wavelength was controlled over 600 pm by the intensity dependence of the refractive index of the photochromic material.