Direct optical observation of walls and disclination effects in active photonic devices

Liquid crystal tunable Bragg Gratings defined in planar substrates via a laser patterning technique exhibit complex wavelength tuning. This tuning displays threshold points and hysteresis. These tuning features are shown to be a manifestation of physical processes occurring in the confined geometry of our tunable devices. Such physical processes include the formation and removal of line disclinations and an associated wall. We discuss the effect of walls in the liquid crystal with regards to voltage tuning characteristics and whether they may allow faster wavelength tuning

Characterization and synthesis of silver nanostructures in rare earth activated GeO2-PbO glass matrix using matrix adjustment thermal reduction method

This paper reports matrix adjustment thermal reduction method to synthesize silver nanostructures in Er3+/Yb3+ activated GeO2-PbO glass matrix. The GeO2-PbO glass, the medium of nanoparticle formation, doped with Er2O3, Yb2O3 and AgNO3 was prepared by a melt quenching method. Annealing of the glass for different times was utilized, not only due to thermally reduce Ag+ ions to Ag nanostructures, but also to influence the glassy network. This is because, the glass structural transformation temperature is near to 435 °C and heating at more than this temperature can cause some structural changes in the glass matrix. According to TEM images, samples that tolerate 450 °C annealing temperature for one hour show the formation of basil-like silver nanostructures with a mean length of 54 nm and mean diameter of 13 nm embedded in the glass matrix, whereas with annealing at 450 °C for 5 to 20 h, silver nanoparticles of about 3–4 nm mean diameter size are formed. Annealing for 30 h causes silver nanoparticles to aggregate to form larger particles due to an Oswald ripening process. Observation of the characteristic Ag-NP SPR band at 400–500 nm in the UV-visible absorption spectra confirms the existence of silver nanoparticles. The SPR band widens to longer wavelengths in one hour annealed samples, which relates to the existence of nanostructures with different size or fractal shapes. In addition, an increment in the peak of the SPR band by increasing the duration of annealing indicates the formation of more nanoparticles. Furthermore, the existence of a peak at 470 cm–1 in the FTIR spectra of annealed samples and its absence in the samples not exposed to an annealing process suggests that the glass matrix is polymerized by Pb-O chains during the 450 °C annealing process. This is the main source of different nanostructures because of the dissimilar stabilizing media. The tighter media cap the particles to form small and dense nanoparticles but a loose environment leads to the creation of basil-like particles in the glass matrix

Multiwavelength hybrid fiber raman/parametric linear oscillator

We demonstrate a linear cavity wideband multiwavelength fiber-based optical parametric oscillator consisting of four fiber Bragg gratings (FBGs). The FBGs center wavelengths are chosen such that they are 3.2 THz (26 nm) and 14.3 THz (115 nm) away from the parametric pump wavelength, with each located in the dominant region of parametric and Raman gain, respectively. Investigation shows that interplay between the lasing processes from the parametric and Raman gain region can be carefully adjusted to produce multiwavelength lasers spanning from 1436 to 1704 nm, with optical signal-to-noise ratio ranging from 14.3 to 54.0 dB

Recent advances in silica glass optical fiber for dosimetry applications

In this paper, we review the highly promising silica glass, fabricated as doped and undoped optical fiber for intended use in radiation dosimetry. The dosimetry techniques reviewed here, underpinned by intrinsic and extrinsic defects in silica glass, focus on Thermoluminescence (TL), Optically Stimulated Luminescence (OSL) and Radioluminescence (RL), with occasional references to the much more established Radiation Induced Attenuation (RIA). The other focus in this review is on the various materials that have been reported earlier as dopants and modifiers used in silica glass optical fiber radiation dosimeters. This article also elaborates on recently reported optical fiber structures, namely, cylindrical fibers, photonic crystal fibers and flat fibers, as well as dimensions and shapes used for optimization of dosimeter performance. The various types of optical fiber radiation dosimeters are subsequently reviewed for various applications ranging from medical dosimetry such as in external beam radiotherapy, brachytherapy and diagnostic imaging, as well as in industrial processing and space dosimetry covering a dynamic dose range from μGy to kGy. Investigated dosimetric characteristics include reproducibility, fading, dose response, reciprocity between luminescence yield to dose-rate and energy dependence. The review is completed by a brief discussion on limitations and future developments in optical fiber radiation dosimetry

Fabrication and development of flat fibers

This paper reports the parameters that affect the fabrication of Flat Fibers, including preform size and doping, furnace temperature, preform feed speed, fiber drawing speed, fiber dimension, fiber quality and shape, vacuum pressure and core dimension. The feed and draw speed generally follows the simplified mass conservation law to draw the fiber to a specific dimension. The preform wall thickness affects the vacuum pressure and furnace temperature that is needed to 'flatten' the fiber. The preform wall thickness is directly proportional to the volume of glass inside the neck-down region. The wall thickness of the preform and its dopant will also affect the size of the cladding and core dimension. Finally, some issues associated with the fabrication of Flat Fibers are also observed and discussed, including fabrication of Flat Fibers with non-uniform dimensions, deformed shapes, unwanted airholes and poor quality of the Flat Fibers

Continuous wave tunable fiber optical parametric oscillator with double-pass pump configuration

We demonstrate a continuous wave tunable fiber optical parametric oscillator in a Fabry–Perot cavity consisting of a 500-m highly nonlinear fiber. In this work, the pump propagates in both directions together with the signal, thus making full use of its parametric gain. The resultant laser peak power is uneven across the wavelength range of interest due to wavelength-dependent phase modulation by the single-mode fiber sections in the cavity. This can be solved by filtering the idler spectral component from the oscillating cavity

UV written waveguide devices - Bragg gratings and applications in sensors

UV direct writing provides a powerful route for the fabrication of integrated optical devices. It has significant benefits in terms of simplicity, flexibility and cost which make it attractive for applying integrated optical concepts to other areas of science and technology. Within our group, we have developed a novel approach for creating Bragg gratings in a planar format, with great flexibility in terms of centre wavelength, bandwidth and position within chip. These planar Bragg grating devices are ideally suited for use in sensors in etched window is created that allows access of the evanescent field of the optical mode to a liquid measurand. The effective index of the mode is altered by the presence of the liquid, and this, in turn, alters the Bragg wavelength of the grating. By measuring the Bragg grating wavelength shift we can resolve refractive index changes of as small 1 part in 10 In this talk, we will present results on sensors for applications in chemical, phase change and biological sensing. Further recent results will show how this sensor configuration may be used with nematic liquid crystals to achieve tunable grating operation with tuning exceeding 100GHz

100 GHz electrically tunable planar Bragg gratings via liquid crystal overlay

We demonstrate 114GHz electrically tunable liquid crystal Bragg gratings using 170Vpp voltage. The devices were made using direct UV grating writing and use evanescent coupling into an electrically tuned nematic liquid crystal. Reconfigurable integrated optical devices are essential in today's dense and complex telecommunication meshes. A commonly employed component on the silica platform fulfilling the above role is a planar Bragg grating. The ability to tune the reflection peak of these gratings is one of the key enablers in realizing an all optical dynamic network. To date, little has been reported on electrically tunable planar Bragg gratings given their potentially superior response times over temperature tuned devices. Such electrically tunable devices work on the principle of shifting the Bragg wavelength by modifying the effective index of a waveguide in a multilayer substrate. One route to achieve this is by overlaying the grating with a liquid crystal as many liquid crystals display refractive index anisotropy that can be electrically manipulated. Modifying the liquid crystal refractive index subsequently alters the effective index of the waveguide, leading to Bragg wavelength shift. Using this approach, Sparrow et al [1] have previously demonstrated 35GHz tunability at 1560nm using 80Vpp (peak-to-peak) square-wave with 250mm-spaced aluminium electrodes. Here, we report a maximum tunability of 114GHz at 1561.8nm using patterned ITO glass electrodes with 170Vpp voltage at 1kHz. Two distinct threshold behaviors which manifest only during the increase of supply voltage were also observed

Liquid crystal based tunable WDM planar Bragg grating devices based on precision sawn groove substrates

Current optical telecommunication systems employ dense Wavelength Division Multiplexing (WDM) techniques to increase the data carrying capacity of fiber networks. Dynamic add/drop and filtering processes are crucial for the precise control of individual channels on these networks. Reconfigurable integrated optical devices, such as planar Bragg gratings, can tune the reflection wavelength over several standard channel spacings, providing the possibility for all-optical dynamic networks. Planar devices have the potential to address and tune several channels simultaneously, and have greater potential for integration than fiber equivalents

The thermoluminescence response of undoped Silica PCF for dosimetry application

This work concerns the suitability of the undoped silica photonic crystal fibre (PCF) as ionizing radiation dosimeters. The dosimetric capabilities of PCF optical fibres in terms of thermoluminescence (TL) and dose response have been investigated and compared with the single mode fibre (SMF) subjected to 6 MeV electron irradiations. The PCF shows dose response linearity curves in a dose range familiar to conventional radiation therapy covering doses from 1 to 4Gy