For medical and analytical applications, thick-core fibers based on synthetic silica are widely spread. In many cases, the fibers are used as a light-guiding medium only; therefore, the coupling efficiency between the light-emitting area and the accepting fiber is of great importance described easily by the light acceptance cone related to the numerical aperture of the fiber. In the past, all-silica fibers with un-doped silica core and fluorine-doped silica cladding have been used for different applications. However, these fibers are restricted in respect to a low numerical aperture of typically 0.22. To increase the numerical aperture, different polymers can be used for cladding material. In addition to standard polymers, Teflon-AF is an attractive candidate for significantly higher NAs of approx. 0.65. In parallel, a new class of all-silica fibers was developed with high NA, the so-called “Air-clad” or microstructured fibers. Longitudinal microstructured holes, in the order of the wavelength, form the cladding-region together with the surrounding silica. The dimensions of the microstructure dictate the critical angle for light transmission in the core, rather than the refractive index of the cladding material. The light guiding properties of different fibers will be compared. Especially the optical transmission from the UV-region up to the NIR-region will be discussed. Due to the wavelength-dependent mean value of the refractive index (RI) in the cladding, the definition of numerical aperture has to be adjusted. Especially, the UV-damage within Teflon-coated fibers and the microstructured fibers will be described in detail.
Optical fibers have been used for data communications in automobiles for several years. The fiber of choice thus far has been a plastic core/plastic clad optical fiber (POF) consisting of the plastic polymethylmethacrylate (PMMA). The POF fiber provides a low cost fiber with relatively easy termination. However, increasing demands regarding temperature performance, transmission losses and bandwidth have pushed the current limits of the POF fiber, and the automotive industry is now moving towards an optical fiber with a silica glass core/plastic clad (PCS). PCS optical fibers have been used successfully in industrial, medical, sensor, military and data communications systems for over two decades. The PCS fiber is now being adapted specifically for automotive use. In the following, the design criteria and design alternatives for the PCS as well as optical, thermal, and mechanical testing results for key automotive parameters are described. The fiber design tested was 200μm synthetic silica core/230μm fluoropolymer cladding/1510μm nylon buffer. Key attributes such as 700 - 900 nm spectral attenuation, 125°C thermal soak, -40 to 125°C thermal cycling, bending losses, mechanical strength, termination capability, and cost are discussed and compared. Overall, a specifically designed PCS fiber is expected to be acceptable for the use in an automotive data bus, and will show improvement in optical transmission, temperature range and bandwidth. However, the final selection of buffer and jacket materials and properties will be most dependent on the selection of a reliable and economical termination method.
Shaped fiber tips are machined or sculpted fiber ends which are formed using the glass from the fiber with no additional glass material. The tips are fabricated through either mechanical or laser machining processes. The tips are very useful in medical and industrial applications which require high power laser delivery (material or tissue cutting), even light distribution over a broad area (tissue ablation or photodynamic therapy), modified beam divergence or spot size (materials processing and communications links), or optical power redirection from the axis of the fiber in areas with small space restrictions (tissue ablation or perforations inside the human body). Descriptions of various shaped tips are provided, with concentration on tapered tips. The tapered tip is the most commonly used. The primary objective of this study was to measure the optical loss of such tapers vs. taper length, input (launch) numerical aperture (NA), and fiber diameter. The tapers fabricated and analyzed were 2:1 tapers using 0.22 NA fibers with 200, 400, and 500 um cores. The optical loss at 633nm for fibers with a 0.22 NA was measured to be 5.9dB (25% transmission) for a fully filled input NA and 0.8 dB (83% transmission) for a 0.12 input NA. The taper loss was found to depend strongly on input NA, but be relatively independent of taper length and fiber diameter. An optical modeling ray trace program was used to analyze the taper performance and validate the actual measurements. The modeling analysis will be a useful tool in design of tapers as well as other shaped fiber tips.
Hollow silica waveguides with internal reflective coatings of silver and silver iodide were tested for optical performance after continuous exposure to 125°C for up to 1300 hours in air. The waveguides were evaluated periodically for mechanical degradation, optical spectral loss, optical loss in bending, and CO2 laser power transmission. The waveguides were found to survive the extended high temperature exposure both mechanically and optically. Mechanically, the internal reflective coatings show no visible signs of deterioration or delamination from the silica tubing substrate. Optically, the waveguides exhibited 1dB/m increase in attenuation at both the 10.6μm and 2.9μm wavelengths for the CO2 and Er:YAG optimized waveguides, respectively. In optical loss in bending, the CO2 optimized waveguides exhibited a 0 -2 dB loss for one 360°, 40cm diameter bend at 10.6μm. The Er:YAG optimized waveguide exhibited higher variability in optical loss in bending and requires further study to determine the true bend loss. The CO2 optimized waveguides were also tested for CO2 (10.6μm) laser power transmission. The change in CO2 laser optical loss pre to post thermal aging was (formula available in book). The post aging waveguides were also shown to transit up to 90W output of CO2 power with no indication of degradation of the internal reflective coatings.
New fiber-optic applications have been demonstrated within the last years, mainly due to the unexpected progress in manufacturing of solarization-reduced fibers. In meantime, analytical systems including UV-fibers and spectrometers are in operation including the wavelength region from 200 to 250 nm.
There are two kinds of `step-index' optical fibers with cores made of pure silica glass--high-OH and low-OH content. High-OH fibers are notable for transparency in the UV region of the spectrum and exhibit high radiation resistance while low-OH fibers are applied in IR applications. Decreasing the Cl2-content in low-OH fibers diminishes the losses in the UV region, which expands the applications for low OH fibers to approximately 300 nm. But the task of fiber production with low OH-content along with transparency in the UV region to 200 nm with high radiation resistance requires a detailed study of the mechanisms of intrinsic and impurity defect formation.
Mainly due to the unexpected progress in manufacturing of solarization-reduced all-silica fibers, new fiber-optic applications in the UV-region are feasible. However, the other components like the UV-sources and the detector- systems have to be improved, too. Especially, the miniaturization is very important fitting to the small-sized fiber-optic assemblies leading to compact and mobile UV- analytical systems. Based on independent improvements in the preform and fiber processing, UV-improved fibers with different properties have been developed. The best UV-fiber for the prosed applications is selectable by its short and long-term spectral behavior, especially in the region from 190 to 350 nm. The spectrum of the UV-source and the power density in the fiber have an influence on the nonlinear transmission and the damaging level; however, hydrogen can reduce the UV-defect concentration. After determining the diffusion processes in the fiber, the UV-lifetime in commercially available all-silica fibers can be predicted. Newest results with light from deuterium-lamps, excimer- lasers and 5th harmonics of Nd:YAG laser will be shown. Many activities are in the field of UV-sources. In addition to new UV-lasers like the Nd:YAG laser at 213 nm, a new low- power deuterium-lamp with smaller dimensions has been introduced last year. Properties of this lamp will be discussed, taking into account some of the application requirements. Finally, some new applications with UV-fiber optics will be shown; especially the TLC-method can be improved significantly, combining a 2-row fiber-array with a diode-array spectrometer optimized for fiber-optics.
All-silica fibers with an undoped core are preferred for UV- applications. There are three different types of synthetic silica for core material, differing mainly in OH-content. Up to now, only high-OH fibers seem to be suitable for UV- applications, because fibers with low-OH core material suffer from pre-existing UV-absorbing color centers due to fiber drawing and generation of these defects during UV-irradiation. With the same loading technique used for commercially available high-OH fibers the amount of initial absorption sites and the generation of color centers, especially the E'- centers, has been reduced in low-OH fibers as well. We studied the influence of this processing on the UV-performance of low- OH all-silica fibers. Besides the E'-centers at 214 and 229 nm, the concentrations of further defects at 245 nm, 265 nm and 330 nm became smaller, too. In addition, a further step of reduction takes place during additional UV-irradiation. After these two steps of improvements, the UV-transmission is in the same range compared to high-OH fiber. This UV-VIS-NIR-fiber shows a broadband transmission from 250 nm to 1.7 micrometer wavelengths, using optimized parameters during the whole multiple-step manufacturing process.
Since several years, UVI-fibers having higher solarization- resistance are well known stimulating new fiber-optic applications in the UV-region below 250 nm. Besides the description of the improved transmission properties of UV- light from different UV-sources, the mechanisms of improvement have been discussed in detail. The UV-defects, mainly the E'- center with the UV-absorption band around 215 nm, were passivated by using hydrogen-doping. Besides DUV-light, ionizing radiation like Gamma-radiation or X-rays can create similar defects in the UV-region. In the past, the radiation- damage in the UV-region was studied on silica bulk samples: again, E'-centers were generated. Up to now, no UV- transmission through a 1 m long fiber during or after Gamma- radiation had been observed. However, the hydrogen in the UVI- fibers behaves the same for Gamma-irradiation, leading to a passivation of the radiation-induced defects and an improved transmission in the UV-C region below 250 nm. On this report, the influence of total dose and fiber diameter on the UV- damage after irradiation will be described and discussed. In addition, we will include annealing studies, with and without UV-light. Based on our results, the standard process of Gamma- sterilization with a total dose of approx. 2 Mrad can be used for UVI-fibers resulting in a good UV-transmission below 320 nm. Excimer-laser light at 308 nm (XeCl) and 248 nm (KrF) and deuterium-lamp light with the full spectrum starting at 200 nm can also be transmitted.