C. P. Gonschior

Silica-based UV-fibers for DUV applications: current status

The current status of UV-damage in several different UV fibers due to defects in their synthetic high-OH silica core and cladding will be described. Further, steps to improve UV resistance and adequate measurement techniques based on a deuterium lamp setup are included. For the first time, the main parameters and their influences on UV induced losses are discussed in detail with an emphasis towards future standardization purposes. Applications based on two new UV light sources, a laser driven xenon plasma broad band source and a high pulse-power 355 nm Nd:YAG laser, are introduced. UV photo-darkening and -bleaching in UV fibers caused by this extremely powerful light source is demonstrated. Finally, first results on transmission of UV light in optical fibers at cryogenic temperatures are shown.

Lifetime prediction for 405-nm single-mode delivery systems for therapeutic laser applications

Near-UV laser light is used for soft tissue treatment for several years now. In first applications the light was delivered directly from the laser, but for in vivo treatment more flexibility was needed. Multi-mode fibers can be used to achieve a high output power coupled from multi-mode lasers. If fiber bundles are used the power can be increased additionally. But the power density on the treated tissue does not rise proportionally, because of the larger spot. A better ablation can be achieved with a Gaussian beam profile coming from a single-mode fiber. Higher beam quality and higher intensity from a small single-mode core produce power densities in the order of kW/cm2 in a focus spot smaller than 100 μm. If the laser therapy is used with the scanning fiber endoscope, treatment in between imaging spirals can be employed and only a single fiber is required. 405 nm laser-induced fluorescence may be able to produce both wide-field fluorescence imaging and laser therapy in a single laser. However additional wavelengths combiners and dual-clad couplers are necessary for multi-wavelength reflectance imaging requiring increased input power to compensate for the losses of these devices. This leads to very high intensities at the fiber coupler and damage will occur at this interface. Differences in damage rate due to differently treated fiber end-faces will be discussed. We suggest a new loss mechanism which is basal for the end-face damage and show miscellaneous methods to reduce the occurring damage and enhance the system lifetime.

Performance of low mode and single mode optical fibers for high peak power 355 nm laser radiation

For applications of fiber guided pulsed UV-laser radiation in biomedical optics, laser spectroscopy or laser micro processing which need good beam quality low mode or single mode optical fibers are required. We investigated the transmission properties at 355 nm wavelength with laser peak powers up to 5 GW/cm2 or laser fluences up to 9.5 J/cm2. In some cases fibers were damaged during prolonged irradiation at this intensity level. So these fluences or intensities can be used as estimation for the damage threshold. It turns out, that degradation or microstructural damage in the fiber core plays a minor role in long term transmission as long as the intensity stays below the damage threshold. Fiber lengths of many meters are possible. Single mode UV laser beam guiding is possible. UV beam guiding with high pulse repetition rate, moderate peak power will be compared with that of moderate repetition frequency, high peak power lasers

Optical fibers in instrumental UV-analytics

Physical and optical properties of optical fibers have improved over recent years significantly. Especially classic UV detection techniques in traditional chemistry, HPLC and dissolution testing rely more and more on fiber optic light guiding techniques to transport light to and from a sample simplifying the design of such detection techniques. An overview on the current status of UV-fiber optical properties will be given in this work. Especially, the reduction of UVdefects in the 215 nm wavelength region leading to a lower drift in the whole system, will be discussed. However, these are not the only parameters of interest in a fiber-optic system. For process control or instrumental analytics, the long-term stability including drift and noise must be determined. This requires stringent fiber test procedures similar to light-sources, connectors and complete detector systems. Further, white-light interference between optical interfaces of a fiber optic detection system due to axial movement, degradation of components and temperature often reduces system stability and must be considered. Finally, a cleaning-in-process of a fiber optic immersion probe will be introduced as a further step of system improvement.

High power 405nm diode laser fiber-coupled single-mode system with high long-term stability

Fiber-coupled 405 nm diode laser systems are rarely used with fiber output powers higher than 50 mW. A quick degradation of fiber-coupled high power modules with wavelengths in the lower range of the visible spectrum is known for several years. Meanwhile, the typical power of single-mode diode lasers around 400 nm is in the order of 100 to 300 mW, leading to single-mode fiber core power densities in the 1 MW/cm2 range. This is three magnitudes of order below the known threshold for optical damage. Our profound investigations on the influence of 405 nm laser light irradiation of single-mode fibers found the growth of periodic surface structures in the form of ripples responsible for the power loss. The ripples are found on the proximal and distal fiber end surfaces, negatively impacting power transmission and beam quality, respectively. Important parameters in the generation of the surface structures are power density, surface roughness and polarization direction. A fiber-coupled high-power 405 nm diode laser system with a high longterm stability will be introduced and described.

Investigation of single-mode fiber output damage by 405nm CW laser light

In the past, the degradation of 405 nm fiber-coupled diode laser systems was investigated in detail with focus on the input end. The coupling and transmission loss of the laser light was associated to the growth of a periodic structure on the input surface. To reduce this damage, a short launch-fiber with a good surface quality was used on the input end surface. Thereby the power transmission was stabilized for at least one month. However, damage structures appeared on the output surface of the single-mode fiber. To investigate this effect, damaged samples were taken after different periods of time and examined with a scanning electron microscope (SEM). Bulges with a submicron periodic structure were found in the core region, too. Additionally, measurements of spectral loss were performed, showing the formation of color centers in the deep UV along the length of the fiber.

Investigation of single-mode fiber degradation by 405-nm continuous-wave laser light

Abstract. The degradation of 405-nm fiber-coupled diode laser systems with more than 50 mW power was investigated in detail with focus on the effects occurring at the input end. The coupling and transmission loss of the laser light were associated with the growth of a projection and a periodic structure on the input surface. To avoid this degradation, a short launch fiber with a good surface quality was used at the input end. In this way, the power transmission was stabilized for at least one month. However, structural degradation was noticed on the output surface of the single-mode fiber. To investigate this effect, the damaged samples were measured after different periods of time and examined with a scanning electron microscope and with an atomic force microscope. Reproducible spherical projections with a submicron periodic structure were found in the core region. Additionally, the spectral loss of the fiber was measured, showing the formation of color centers in the deep ultraviolet along the length of the fiber. These investigations were accompanied by simulations of the growth of the structure on the output surface. The influence of the structure was mainly on the divergence angle of the emitted laser beam, reducing the beam quality for applications.

Optical fibers for 355nm pulsed lasers and high-power broadband light sources

In the past, the spectral stability of multimode UV-fibers has been mainly characterized using deuterium lamps with a broadband spectrum in the DUV. In meantime, new UV light-sources with higher powers are available. For example, improved pulsed Nd:YAG lasers with higher harmonics or high-power broadband plasma lamps are interesting candidates for new systems and applications. Because of better beam quality, multimode all-silica fibers with core diameter smaller than 100 μm can be recommended. A new step-index fiber with a large cladding-to-core ratio will be introduced. Using the new light-sources, the degradation during UV-light delivery will be described in detail, comparing the hydrogen-loaded and non-loaded version of this fiber. These results of UV-induced damage will be compared to a commercially available improved 100 μm UV-fiber damaged with deuterium lamp.

Evanescent-field sensor using selectively excited modes in step-index fibers

High-order skew modes will be excited in multimode step-index fibers using special excitation conditions. As a result, light with an angle of incidence larger than the maximum angle for meridional modes given by the numerical aperture of the fiber can be coupled into a fiber. Combining the selective mode-excitation with new powerful broadband light-sources, the spectral light-guidance of such skew modes in different optical fibers will be described in detail. Results of the proposed system in context of different light-sources will be discussed. A new evanescent sensor approach based on controlled coupling of skew modes will be introduced. Finally, first steps to construct such sensors for medical and analytical applications will be presented.