We have developed an advanced design for a dual-wavelength laser source used in Coherent Anti-Stokes Raman scattering (CARS) microscopy and Stimulated Raman Spectroscopy (SRS) addressing a wider frequency range for bio-imaging, spanning from <630cm^-1 to 2250cm^-1, while maintaining a fast tuning time of a few seconds across the entire range. Furthermore, we have achieved improved tunability of the Stokes and pump power, allowing for versatile applications. The system supports repetition rates of up to 10MHz, and the spectral width of the signals is narrow (<15cm^-1), enabling a wide range of potential uses.
Fiber-based laser sources for Coherent Anti-Stokes Raman scattering (CARS) microscopy and Stimulated Raman Spectroscopy (SRS) have been successfully used for label-free bio-imaging in the past. We present an improved design of a dual-wavelength source for CARS and SRS enabling a wider frequency-range from <630cm^-1 up to 2250cm^-1 while the tuning time remains in the range of a few seconds over the full range. The system is fiber-based and thus resistant to detrimental thermal effects even at output powers in the Watt-range with improved tunability of Stokes and pump power. Repetition rates up to 10MHz are possible and the spectral width of the signals is <15cm^-1 enabling most applications.
We present a sub-2-cycle laser system combining high average power, pulse energy and repetition rate with CEP-stable operation. The laser system creates 300 fs pulses with 1.8 mJ pulse energy that are nonlinearly post-compressed down to few optical cycles in two subsequent multipass cells (MPC). A pulse duration of 5.8fs (sub-2-cycle) at a pulse energy of 1.1mJ in combination with 110W average power (100 kHz) is achieved. This corresponds to the shortest pulses and highest compressed average power for few-cycle MPCs. Furthermore, the carrier-to-envelope-phase stability amounts to 300 mrad for frequencies above 2 kHz as measured by stereo—above-threshold-ionization (ATI).
Fiber-based laser sources for Coherent Anti-Stokes Raman scattering (CARS) microscopy and Stimulated Raman Spectroscopy (SRS) have been successfully used for label-free bio-imaging in the last few years. However, these systems were limited so far to a few experimental setups in university labs. We present the first extremely compact commercially available dual-wavelength source for CARS and SRS. With a foot-print of <30cm x 50cm this alignment-free turn-key system offers tuning over the highly relevant frequency range from <500cm^-1 up to 5000cm^-1 within a few seconds without any detrimental thermal effects. The spectral width of the signals is <10cm^-1 enabling a multitude of applications.
Among other modern imaging techniques, stimulated Raman Scattering (SRS) requires an extremely quiet, widely wavelength tunable laser, which, up to now, is unheard of in fiber laser systems. We present a compact all-fiber laser system, which features an optical parametric oscillator (OPO) based on degenerate four-wave mixing (FWM) in an endlessly single-mode photonic-crystal fiber. We employ an all-fiber frequency and repetition rate tunable laser in order to enable wideband conversion in the linear OPO cavity arrangement, the signal and idler radiation can be tuned between 764 and 960 nm and 1164 and 1552 nm at 9.5 MHz. Thus, all biochemically relevant Raman shifts between 922 and 3322 cm-1 may be addressed in combination with a secondary output, which is tunable between 1024 and 1052 nm. This ultra-low noise output emits synchronized pulses with twice the repetition rate to enable SRS imaging. We measure the relative intensity noise of this output beam at 9.5 MHz to be between -145 and -148 dBc, which is low enough to enable high-speed SRS imaging with a good signal-to-noise ratio. The laser system is computer controlled to access a certain energy differences within one second. Combining FWM based conversion, with all-fiber Yb-based fiber lasers enables the construction of the first automated, turn-key and widely tunable fiber laser. This laser concept could be the missing piece to establish CRS imaging as a reliable guiding tool for clinical diagnostics and surgical guidance.
XLMA fibers based on Yb-doped bulk silica possess an excellent refractive index and doping level homogeneity [1]. To
achieve the highest optical-to-optical efficiency and long-term operation without degradation we simulated the effect of the brightness conversion factor of different core dopant compositions of such XLMA fibers. We also investigated the beam quality of a multi-kW single XLMA fiber laser system and its long-term stability. The current state-of-the-art
XLMA single fiber laser has 5 kW maximum output power and a degradation rate of about 0.5 % / 500 h at 4 kW
measured over a period of 1700 h. Several application tests demonstrate the excellent performance of the XLMA fiber
laser.
The properties of optical fibers can significantly be influenced by intrinsic stress. It is well known that these
stresses are caused by various reasons, e.g. the variations in the thermal expansion coecient of the differently
doped regions in the fiber. The so called thermal stresses are only dependent on the composition of the fiber
and not on its preparation history. Another main reason for stress in the final fiber is the mechanical force
that is applied during the fiber drawing process. It generates so called mechanical stress that depends on the
composition of the fiber and the thermal history of the fiber fabrication process.
Using a non-destructive polarimetric system, we are able to measure the intrinsic stress state in optical fibers
as well as in their preforms. Knowing on the one hand the thermal induced stresses in the preform of a fiber and
on the other hand the final stress state in the fiber itself, we are able to differentiate between the two kinds of
stress.
In this paper we present results of stress measurements on optical ber preforms and fibers. We show, that
the measured stress profile in the preform matches the theoretically assumed stress profile for thermal stress
very well. Moreover we used this preform to draw fibers under different drawing conditions represented in a
large difference in the applied force during the fiber drawing. We present the stress results for these differently
fabricated fibers and show how huge the effect of the drawing tension can be. We find that for high drawing
forces, the stress state can be reversed in comparison to the thermal stresses that are induced by the material
composition. Due to the fact that stress on the one hand has a strong effect on the mechanical properties of glass
and modifies the refractive index, this can lead to signicant effects on the fiber stability and modal behaviour.
Finally, we present a way to compensate the additionally induced mechanical stress, which is for example
a very good possibility to increase the stress birefringence in polarization maintaining (PM) fibers with panda
structure. We compare the mechanical stress states of such Panda Fibers after their fabrication with the state
after an additional high temperature step. We clearly find that it is possible to improve the birefringence of
these fibers using appropriate preparation steps.
The quality of Yb-doped fused bulk silica produced by sintering of Yb-doped fused silica granulates has improved
greatly in the past five years [1 - 4]. In particular, the refractive index and doping level homogeneity of such materials
are excellent and we achieved excellent background fiber attenuation of the active core material down to about 20 dB/km
at 1200 nm. The improvement of the Yb-doped fused bulk silica has enabled the development of multi-kW fiber laser
systems based on a single extra large multimode laser fiber (XLMA fiber).
When a single active fiber is used in combination with the XLMA multimode fiber of 1200 μm diameter simple and
robust high power fiber laser setups without complex fiber coupling and fiber combiner systems become possible. In this
papper, we will discuss in detail the development of the core material based on Yb-doped bulk silica and the
characterization of Yb-doped fibers with different core compositions.
We will also report on the excellent performance of a 4 kW fiber laser based on a single XLMA-fiber and show the first
experimental welding results of steel sheets achieved with such a laser.
Silica based microstructured holey fibers offer the possibility for filling with unconventional fiber materials. Of
special interest are chalcogenide glasses due to their high refractive index and their nonlinear optical properties.
We demonstrate two types of fibers: an index guiding fiber type with high-index glass core and silica cladding and
a fiber with silica core surrounded by a periodic, hexagonal high-index glass structure giving antiresonant guiding
properties. We prepared such fibers filled with arsenic sulphide glass and arsenic selenide glass by a pressurized
infiltration technique. The manufacturing process is modelled on the basis of viscous glass flow parameters and
is compared with experimental results obtained from the filled fibers. The propagation and spectral transmission
properties of such fibers are measured and discussed.
Sintering of Yb-doped fused silica granulates is a well established technique developed by the IPHT and Heraeus
Quarzglas and it produces very homogeneous rare earth doped bulk silica core rods for fiber laser applications. By using
a newly developed laser induced deflection (LID) technique we are able to pre-characterize directly the material
absorption properties of the bulk material prior to the laser fiber production. The bulk absorption results measured by
LID are without scattering effects and they are typically in good agreement with the total attenuation measured in the
fibers. We achieved a fiber background loss of 20 dB/km. Furthermore, we present detailed studies of the refractive
index homogeneity of the Yb-doped bulk materials and laser fibers to show the unique features of the Yb-doped bulk
silica.
Multimode double cladding laser fibers with an extra large mode area XLMA fiber design (core diameter up to 100 μm)
have been produced from the Yb-doped bulk silica rods by two different techniques. One is a classical jacketing method;
the other employs the stacking of un-doped, Yb- and F-doped rods and F-doped tubes.
Different fiber types have been tested in different fiber laser setups. The influence of the fiber end cap properties on the
fiber laser focus shift is discussed in detail. We have achieved fiber laser output powers up to 1.925 kW, limited only by
the pump power. We also investigated the long term laser stability at different power levels.
It is well known that thermal stress can significantly influence the properties of optical fibers. These stresses are caused
by variations in the coefficient of thermal expansion (CTE) of the differently doped areas in the fiber, like the core and
the cladding. On the one hand, the stress has a strong effect on the mechanical stability of fibers. On the other hand, the
stress also modifies the most essential property of a fiber, the refractive index distribution, and therefore also the
propagation properties. Similar to the effect of generation of birefringence in polarisation maintaining fibers, thermal
stress also generates changes in the refractive index of the differently doped regions in the fiber.
We report on results of non-destructive polarimetric stress measurements in ytterbium doped fiber preforms, that are
codoped with aluminum as well as with phosphorus. Simple models of changes in the CTE for samples doped with
multiple elements assume an additive superposition of the changes caused by each dopant. In contrast to such simple
models, our investigations have shown that the induced stress cannot be explained by an additive change in the CTE of
the glass material. The occurring stresses turn out to be smaller than the simple sum of the effects generated by the
respective dopants. This result is also in agreement with measurements of the refractive index profile of these samples.
The changes in the index are again not additive for doping with both aluminum and phosphorus.
At the Photonics West 2008 we presented our rare earth doped fused bulk silica for fiber laser applications [1]. This
approach overcame the typical geometrical limitations of other well known production methods for rare earth doped
silica materials. Our unique production technique is based on the sintering of Yb-doped granulates of high-purity SiO2
particles. We have processed our Yb-doped bulk silica rods into ultra large mode area (XLMA) multi-mode double
cladding laser fibers with an active core diameter in the range of 40 μm to 100 μm (depending on the core doping level).
In the XLMA fiber the active core is surrounded by a so-called 2D- or 4D-shaped pure silica pump cladding (with
diameter between 850 μm and 900 μm) and an F-doped outer silica cladding with an outer diameter of 1000 μm.
We have investigated the refractive index and the intrinsic stress profiles of different XLMA laser fibers and their
preforms to visualize interface effects. The fiber cross section designs, the quality of all interfaces and the material
composition are important factors for the laser fiber performance. The laser properties of these fibers have been
investigated in detail. In addition, the preparation of the fiber end-face is important to reduce heat effects and we have
developed concepts to mitigate such thermal load at the fiber end face.
Mechanical stress can significantly influence the optical and mechanical properties of fibers. We present results of
nondestructive measurements of thermal stress in rare-earth-doped optical fibers, which are related to the doping
variations across the fiber cross section and the preparation conditions. The samples are preforms and fibers with a large
ytterbium doped core that were prepared by an alternative glass forming process, which is capable to realize bigger ratios
of core to cladding diameter than the MCVD-method. To the best of our knowledge, this is the first time measurements
of stresses and birefringence were made on such active optical fibers and their preforms. The results are compared to
previous results on ytterbium-doped samples fabricated by the MCVD-technology. From the resultant stress profile, we
could calculate the stress induced index changes that are relevant for the light propagation in optical fibers, and discuss
the reasons for the observed mechanical stress. In addition, we could evaluate the samples regarding their changes in
thermal expansion coefficient induced by doping with one or more oxides in several concentrations and different ratios.
We confirmed our earlier result that the active doping component ytterbium generates much higher stress than other
common dopants like aluminum, phosphorus or boron.
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