Kevin Cook

Extreme Silica Optical Fibre Gratings

A regenerated optical fibre Bragg grating that survives temperature cycling up to 1,295°C is demonstrated. A model based on seeded crystallisation or amorphisation is proposed.

Anatomy of a femtosecond laser processed silica waveguide [Invited]

Waveguides are inscribed through densification of the surrounding region of a damage induced channel created by femtosecond irradiation within silica. Single mode propagation at 1.5 μm is obtained below the damage region whilst at shorter wavelengths guidance is only observed away on either side of the region. The quasi-periodic nanostructure that is induced can explain the mode profile elongation observed with polarised light at 45°. The origin of this guidance area is explored using SEM analysis, which reveals nanoporous regions within laser track structure above and below the densified region where 1.5 μm propagates. Shorter wavelength light is not supported in this area.

Delivery of sub-100fs pulses through 8m of hollow-core fiber using soliton compression

We report soliton compression in a tapered hollow-core photonic bandgap fiber. We compress unchirped 195fs input pulses at 800 nm wavelength to less than 100fs after single-mode propagation through 8m of fiber, at pulse energies of around 50nJ.

Regeneration and helium: regenerating Bragg gratings in helium-loaded germanosilicate optical fibre

We have demonstrated successful regeneration of optical fibre Bragg gratings that have been loaded with helium as opposed to hydrogen. The high temperature stability of these gratings is shown to be comparable to the gratings regenerated using hydrogen – surviving temperatures in excess of 900 °C for over 4 hours. These results using an inert gas confirm our previous model where mechanical relaxations dominate regeneration. Consistent with this, He is also observed to play no local role in changing index modulation whilst increasing average index change during grating writing.

Cleaving of Extremely Porous Polymer Fibers

Different cleaving techniques, based on the use of a semiconductor dicing saw, focused-ion-beam milling, and a 193-nm ultraviolet laser, have been exploited to cleave highly porous polymer fibers developed for guiding terahertz radiation. Porous fibers made up of two different polymer materials have been cleaved with the proposed methods and compared with those achieved from the conventional cleaving method. Regardless of the polymer material used for fabricating terahertz porous fibers, using an ultraviolet laser for cleaving and rotating the fiber during the process rapidly provides smooth and reproducible cleaves across the entire fiber cross section.

Gratings in Structured Optical Fibres

Grating writing in structured optical fibres and their properties and applications are reviewed. To date, most gratings have been written in a straightforward manner into structured fibres containing a photosensitive germanosilicate step-index core. However, gratings have also been written directly into single material, structured silica fibres and into air-clad cores using two and higher-photon processes with both UV and near IR pulsed (nanosecond-femtosecond) light. Given the intrinsic-added functionality possible within a structured optical fibre, structured fibre gratings offer further capabilities for sensors, diagnostics, lasers, and devices.

Thermal stabilization of Type I fiber Bragg gratings for operation up to 600°C

The thermal stability of Type I gratings is increased by postthermal tuning of the grating. Optimization of the procedure leads to gratings that can withstand temperatures as high as 600 degrees C. Aging tests lead to lifetime predictions as high as 25 years with <3 dB reduction at 400 degrees C. Single exponential relaxation is observed. Above 800 degrees C regeneration is obtained.

Regenerated Fibre Bragg Gratings

Silica remains the key optoelectronic and photonic medium, the essence of nearly all modern optical transport systems. Engineering of silica in its various forms ranges from 1 to 3-dimensional waveguide and periodic structures, including recent interest in 3-D photonic crystals. Most of the processing methods involve complex vapour deposition and various co-dopants, which have an advantage of overcoming the lack of finesse involved with general formation of glass structure through high temperature processing and quenching. Nevertheless, to obtain micron or sub-micron precision over the processing of glass for device purposes, invariably post processing methods are commonly used, ranging from etching of systems with dopants, often through patterned masks, to laser processing using UV to mid IR lasers. Concrete examples of micron scale laser processing of glass include direct written waveguides, Bragg gratings in waveguides and optical fibres and photonic crystals. The drawback with these post-processing techniques is that they often produce glass that is structurally less stable than the starting phase. For many applications the thermal stability of laser induced glass changes determines the limits in which they can operate – an excellent example which will form the basis for this chapter, is the optical fibre Bragg grating. Fibre Bragg gratings are used in many industrial and technological applications. Within standard telecommunications applications, for example, type I fibre Bragg gratings that can operate to 80°C for 25 years are required – such gratings can in principle operate for lengthy periods up to 300°C. Gratings that can operate at temperatures well above standard telecommunication requirements are critical to the success of many real time sensing applications. In the oil and gas industries, an alternative application, although standard oil bores are typically quoted as having an environment no more than ~(180-250)°C [Schroeder et al. 1999; Kersey 2000], variations can occur and the increasing depth of the next generation bores suggest sensors that can operate to 400°C or more are desirable for long term or permanent operation. In industries involving high temperature furnaces, such as aluminium smelting or coal based power stations, it would be of interest to be able to monitor temperatures in excess of 1000°C. Similar temperature requirements span many

Air-structured optical fiber drawn from a 3D-printed preform.

A structured optical fiber is drawn from a 3D-printed structured preform. Preforms containing a single ring of holes around the core are fabricated using filament made from a modified butadiene polymer. More broadly, 3D printers capable of processing soft glasses, silica, and other materials are likely to come on line in the not-so-distant future. 3D printing of optical preforms signals a new milestone in optical fiber manufacture.

Optical fiber smartphone spectrometer.

An optical fiber-based smartphone spectrometer incorporating an endoscopic fiber bundle is demonstrated. The endoscope allows transmission of the smartphone camera LED light to a sample, removing complications from varying background illumination. The reflected spectra collected from a surface or interface is dispersed onto the camera CMOS using a reflecting diffraction grating. A spectral resolution as low as δλ∼2.0  nm over a bandwidth of Δλ∼250  nm is obtained using a slit width, ωslit=0.7  mm. The instrument has vast potential in a number of industrial applications including agricultural produce analysis. Spectral analysis of apples shows straightforward measurement of the pigments anthocyanins, carotenoid, and chlorophyll, all of which decrease with increasing storage time.