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.
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.
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.
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.
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/cm<sup>2</sup> 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.
Surface and bulk effects in silica optics due to high intensity laser light are well known using short pulse and high power
laser systems. Surfaces are quickly destroyed mechanically if not properly prepared and thoroughly cleaned. Linear and
non-linear absorption of high intensity laser light in the bulk of the optics causes material modifications, like voids,
cracks and UV defects. In ablation experiments with very short pulses on wide band-gap dielectrics, periodic surface
structures in the form of ripples were found. Surprisingly, we found similar structures on fiber end-faces after long-term
irradiation with 405 nm CW laser light. Power densities on the end-face are in the range of 1 MW/cm<sup>2</sup>, three magnitudes
of order below the power threshold at which the described damages occur. Nevertheless a ripple structure perpendicular
to the polarization direction of the laser was formed and grows with irradiation time. An increased absorption band at
214 nm (E' center) along the fiber was discovered by spectral absorption measurements. E' centers can be generated by
405 nm laser light in the bulk, therefore defects on the surface are possible as well. The generation of defect centers on
the silica surface can enhance the formation of an unstable surface layer.
As the demand for high power fiber-coupled violet laser systems increases existing problems remain. The typical power
of commercially available diode lasers around 400 nm is in the order of 100 to 300 mW, depending on the type of laser.
But in combination with the small core of single-mode fibers reduced spot sizes are needed for good coupling
efficiencies, leading to power densities in the MW/cm<sup>2</sup> range. We investigated the influence of 405 nm laser light
irradiation on different fused silica fibers and differently treated end-faces. The effect of glued-and-polished, cleaved-and-clamped and of cleaved-and-fusion-arc-treated fiber end-faces on the damage rate and behavior are presented. In
addition, effects in the deep ultra-violet were determined spectrally using newest spectrometer technology, allowing the
measurement of color centers around 200 nm in small core fibers. Periodic surface structures were found on the proximal
end-faces and were investigated concerning generation control parameters and composition. The used fiber types range
from low-mode fiber to single-mode and polarization-maintaining fiber. For this investigation 405 nm single-mode or
multi-mode diode lasers with 150 mW or 300 mW, respectively, were employed.
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/cm<sup>2</sup> 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
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.
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/cm<sup>2</sup> or laser fluences up to 9.5 J/cm<sup>2</sup>.
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
There is an increasing demand for UV single-mode fibers with long-term stability. Due to the small diameter, the
coupling efficiency is extremely low, if broadband light-sources or LEDs with near field diameter in the order of approx.
500 μm are used. Therefore, UV lasers are the real candidates for these fibers. Although the power is in the order of
several milliwatts, the intensity in the small fiber core is significantly high. Therefore, UV damage has to be taken into
account, even in the wavelength region above 300 nm. After the introduction of adjusted measurement systems for these
fibers, the properties of some low-mode and single-mode fibers will be shown and discussed.
In step-index multimode-fibers, controlled excitation conditions are essential to achieve optimized transmission
properties and control the output beam profile. Especially with lasers as a light-source, selective methods of mode
excitation can be used easily.
So far, fiber properties have been specified using meridional rays (modes), as proposed with the inverse far-field method.
In addition to these meridional rays in step-index fibers, high-order skew rays can be selectively excited. Especially with
excitation angles higher than the numerical aperture of the fiber, these skew rays can propagate with interesting
properties. Based on these extreme test conditions, the core-cladding interface of large-core step-index fibers can be
more efficiently controlled.
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.
Due to the doubling of the internet traffic every twelve month and upgrading existing optical metro-, regio- and long haul
transport networks, the migration from existing networks toward high speed optical networks with channel data rates up
to 100 Gbit/s/λ is one of the most important questions today and in the near future. Current WDM Systems in photonic
networks are commonly operated at linerates of 2.5 and 10 Gbit/s/λ and major carriers already started the deployment of
40 Gbit/s/λ services. Due to the inherent increase of the bandwidth per channel, limitations due to linear and non-linear
transmission impairments become stronger resulting in a highly increased complexity of link engineering, potentially
increasing the operational expenditures (OPEX). Researchers, system vendors and -operators focus on investigations,
targeting the relaxation of constraints for 100 Gbit/s transmission to find the most efficient upgrade strategies.
The approaches towards increased robustness against signal distortions are the transmission of the 100 Gbit/s data signals
via multiple fibers, wavelength, subcarriers or the introduction of more advanced modulation formats. Different
modulation schemes and reduced baud rates show strongly different optical WDM transmission characteristics. The
choice of the appropriate format does not only depend on the technical requirements, but also on economical
considerations as an increased transmitter- and receiver-complexity will drive the transponder price.
This article presents investigations on different approaches for the upgrade of existing metro-/ regio and long haul
transport networks. The robustness against the main degrading physical effects and economy of scale are considered for
different mitigation strategies.
UV solarization resistance of synthetic silica/silica fibers has been researched over many years. Fiber optic probes for
applications as diverse as protein analysis, dissolution testing or high pressure liquid chromatography have been
developed and successfully commercialized. Although fabrication technology for optical fibers has improved
significantly and optical losses due to solarization effects have been minimized in synthetic silica fibers, the generation
of UV induced defects in silica fibers due to the generation of E'centers visible in the 215 nm region is still present and
can interfere with sensitive spectroscopic absorbance measurements. This work presents methodology to determine the
transient response of optical fibers in the 200 nm to 300 nm region during the warm up period and during measurement
as a function of light power coupled into the fiber, fiber length and fiber diameter.