Fiber Bragg gratings (FBG) arrays in silica based optical fibers are increasingly used in applications involving system monitoring in extreme high temperature environments. Where operational temperatures are < 600 °C, traditional UVlaser inscribed FBGs are not appropriate since the induced Type I index change is erased. Instead two competing FBG technologies exist: 1) regenerative FBGs resulting from high temperature annealing of a UV-laser written grating in a hydrogen loaded fiber and 2) FBGs written with femtosecond infrared pulse duration radiation (fs-IR), either using the point-by-point method or using the phase mask approach. Regenerative gratings possess low reflectivity and are cumbersome to produce, requiring high temperature processing in an oxygen free environment. Multiple pulse Type II femtosecond IR laser induced gratings made with a phase mask, while having very good thermal stability, also tend to have high insertion loss (~ 1dB/grating) limiting the number of gratings that can be concatenated in a sensor array. Recently it has been shown that during multiple pulse type II thermally stable fs-IR FBG production, two competing process occur: an initial induced fs-IR type I FBG followed by a thermally stable high insertion loss type II FBG. In this paper, we show that if only a type I FBG is written using type II intensity conditions but limited numbers of pulses and then annealed above 600 °C, the process results in a type II grating that is stable up to 1000 °C with very low insertion loss ideal for an FBG sensor array.
With the proper choice of laser parameters focused femtosecond laser light creates long-range self-assembled planar nanocracks inside and on the surface of fused silica glass. The orientation of the crack planes is normal to the laser polarization direction and can be precisely controlled. The arrays of cracks when properly oriented and combined with chemical etching produce high aspect ratio micro- and nanofluidic channels. Direct femtosecond laser writing without any chemical etching can be used to fabricate embedded nanoporous capillaries in bulk fused silica for biofiltering and electrophoresis applications. The morphology of the porous structures critically depends on the laser polarization and pulse energy and can be used to control the transmission rates of fluids through the capillaries. Finally high aspect ratio, polarization-dependent, self-ordered periodic nanoslots can be fabricated from nanocracks produced on the surface of fused silica wafers. Control of the surface slot width from 10 to 60 nm is achieved through selective chemical etching. This technique, which may be useful for Surface Enhanced Raman Scattering (SERS) applications, has sub-diffraction limited resolution and features high throughput writing over centimeters.
Multimode fiber (MMF) has found applications in high-speed computer interconnect, local area networks (LAN), and storage area networks (SAN) due to its ease of handling and high performance over short span. However, modal dispersion limits its bandwidth-distance product (BDP) to about 2 Gb/s-km. This limit has been extended by recent new generation of optimized MMF to 28 Gb/s-km, but there is evidence that a substantial portion of installed MMF have imperfect refractive index (RI) profiles due to defects during the manufacturing process, and the BDP might be at best no more than 500 Mbps-km. Different strategies have been proposed to address this issue by employing offset launch, multi-level subcarrier modulation, and mode spatial control. However, our studies have shown that end-to-end system performance of installed MMF can be highly dependent on input launch polarization. In this report, we investigate, for the first time to our knowledge, the relationship between RI profile defect, input launch condition, and transmission performance in commercial-grade MMF, both 50 μm and 62.5 μm. To this end, a number of techniques have been deployed. Two-dimensional (2D) MMF RI profile is obtained by a micro-reflectivity technique with a spatial resolution of ~400 nm. MMF transmission characteristics are interrogated using interferometric techniques. Data at 40 Gb/s are transmitted over the same MMF sample at different launch conditions, and the system performance is evaluated by bit-error rate measurements. These results are then analyzed to provide insights to correlate fiber RI profile defects and high-speed data transmission performance for installed commercial-grade MMF for optical access networks.
Self-organized nanostructures have been recently observed when femtosecond laser pulses were focused inside fused silica glass. We have shown that these nanostructures extend throughout the focal volume and their order is preserved over macroscopic distances when the focus is scanned. We discuss the present understanding of the formation of the nanostructures including a model based on transient nanoplasmonics. The model predicts the periodicity of nanoplanes to scale as λ/2 in the medium. This is experimentally verified at 800 nm and 400 nm light with which we obtain nanoplane spacing of 250 ± 20 nm and 140 ± 20 nm respectively, which scale as predicted. Another requirement of the model is that ionization occurs preferentially at regions that have previously been ionized. This allows an initially inhomogeneous plasma to develop into an ordered nanoplasma array. Using transmission measurements we show that the required "memory" exists in the case of fused silica.
A comparison is made between three high spatial resolution index of refraction profiling techniques:reflection-NSOM,
microreflection and AFM plus selective chemical etching using the very small elliptical core of a polarization maintaining E-fiber from Andrew Corporation as a test waveguide.
The energetic 7.9-eV photons of the F<sub>2</sub> laser directly access bandgap states in germanosilicate glasses to provide a strong and direct channel for inducing refractive index changes in optical fibers and planar waveguides. In this paper, we review our F<sub>2</sub>-laser photosensitivity studies with an aim to assess prospects for shaping useful photonics structures directly inside the germanosilicate waveguides. We describe strong photosensitivity responses in standard telecommunication fibers and planar optical waveguides without the need for hydrogen loading, and compare with responses provided by traditional ultraviolet lasers. Because of the strong 157-nm absorption in the germanium-doped guiding layers, large non-uniform changes to refractive index are noted that offer opportunities for trimming phase errors and correcting waveguide birefringence in planar optical circuits. With hydrogen soaking, modest 157-nm pre-irradiation was found to 'lock-in' a permanent photosensitivity enhancement in the germanosilicate guiding core, permitting the formation of strong (40-dB) and stable fiber Bragg gratings with 248-nm KrF laser light. The 157-nm 'lock-in' mechanism is associated with Si-OH and Ge-OH defect formation and permanently enhances the ultraviolet photosensitivity response by several orders of magnitude above that for an untreated fiber without the aging related disadvantages of conventional hydrogen soaking. The unique opportunities for F<sub>2</sub>-laser photosensitivity applications in shaping and trimming photonic components will be outlined in this presentation.