We investigate a new phenomenon, where a reciprocal fiber ring laser switches from bidirectional to unidirectional operation above a certain pump power threshold. We present significant simplifications regarding earlier experiments, which for the first time allow the identification of individual nonlinear effects. We highlight the unique role of stimulated Raman scattering in triggering unidirectional operation.
Fiber lasers have become very popular in the recent two decades due to numerous advantages compared to other solid-state lasers, e.g. their robustness, efficient cooling, high power compatibility, flexibility, large gain bandwidth and comfortable handling. Visible fiber lasers are increasingly becoming a new research focus, since they can be pumped very efficiently with new powerful, commercially available high-power GaN diodes in the UV wavelength range. Potential and important applications, especially in the biomedical field, are spectroscopy, microscopy and microsurgery. So far, fluoride glasses, e.g. ZBLAN, InF and AlF, are the most promising host glasses to realize stable visible fiber lasers. Most of them are originally designed for the MIR spectral range, but they also have the potential to cover the VIS. Pr3+ in ZBLAN glass is very attractive as it offers many possible laser transitions in the VIS and NIR range. In [1] a wide tunability in the VIS and NIR is demonstrated by core-pumping of a very short Pr3+-doped ZBLAN fiber. Regarding the visible spectral range, laser output powers up to 2.3 W around 635 nm were reported recently in a monolithic ZBLAN fiber laser with a Fiber- Bragg grating, but with reduced slope efficiency of 14% [2]. An alternative approach, where authors demonstrated up to 5 W laser power and 25.7% efficiency around 635 nm, is the direct and spliceless connection of pump and laser fiber using an end-facet coating mirror as input coupler [3]. Using this approach emission wavelength is power dependent and not locked.
We have investigated the laser performance and tunability of a highly Pr3+ doped fluoride glass fiber. Absorption and emission properties as well as laser characteristics in different resonator configurations have been studied. We have achieved 1.3 W output power @ 635nm wavelength with a slope efficiency of 30.6% with respect to the incident pump power. Furthermore, to the best of our knowledge, we present results on wavelength tunability with cladding-pumping on a several meter long Pr3+-doped ZBLAN fiber for the first time.
We recently reported on a new form of unidirectional lasing in a fully reciprocal kilometre long fiber ring cavity. It is a threshold like phenomenon where unidirectionality is triggered at full laser operation far above the laser threshold. Directionality is the consequence of asymmetric round trip loss, which arises due to differing nonlinear loss because of Raman scattering for the two directions. Asymmetry can be altered by seeding any direction at Stokes wavelengths to define the final direction of the laser. This was achieved by a simple straight cleaves at WDM outputs, used to tailor the excited spectrum.
We report on the observation of a new phenomenon, occurring in a fiber ring laser. This phenomenon describes the transition from an initially bidirectional emission of a reciprocal fiber ring laser to a unidirectional emission at a certain pump power threshold. In addition, the final direction is not predefined but appears to be randomly chosen every time the threshold is exceeded. Therefore, we term this new phenomenon as direction instability. In addition we provide a first discussion of how the pump power threshold and the final direction can be influenced by the length and the loss of the cavity.
Fiber lasers are a great source for tunable lasers due to the wide and relatively flat gain spectra of rare earth transitions in a glassy host (as compared to crystals). Thulium (Tm)-doped fibers, in particular, offer an extremely wide tunability of up to 330 nm in the 2μm wavelength region in a dual gain module configuration1. More recently, new concepts have emerged, which allow the synchronized emission of two or even more wavelengths2. These sources are particularly useful for nonlinear frequency conversion via four-wave mixing (FWM) or difference frequency generation (DFG). We will present a very versatile fiber-integrated approach based on Fiber-Bragg-Grating (FBG) arrays implemented in a theta-shaped cavity. The Tm-doped fiber source emits typical average powers of 0.5W and is tunable from 1931nm to 2040nm. The emission linewidth follows the spectral characteristic of the FBG and is typically 30GHz in our case. This concept allows a constant wavelength-independent repetition rate as well as a synchronous emission of two or even three independently tunable wavelengths. The tuning is performed purely electronically by optical gating, and in addition the pulse duration can be tuned between 4ns and 25ns. The switching speed is very fast and was measured to be less than 10μs. These experiments will be contrasted with a different approach based on a VLMA fiber associated to a set of two volume Bragg gratings (VBG), one of them being angle-tunable. This concept allows pulsed (Q-switched) as well as CW operation and features a continuous and wider tunability of up to 144nm especially and also the dual wavelength mode. The output power was > 4.5W in CW mode and pulse peak power of 12kW have been obtained in the Q-switched mode with pulse durations of 25ns.
We report on OH diffusion effects at preparation of hollow core fibers with large central hexagonal and square shaped cores. The fibers with up to 30 μm central hole diameter are suitable for chemical sensor applications, especially for gases. We demonstrate the single mode guidance at selective bands in the UV, VIS and NIR region. Key feature of low loss in specific spectral windows of such fiber structures is the control of thickness and homogeneity of the web bridges. The fibers achieve a minimum loss of 2 dB/m and effective single mode propagation in the wavelength range between 270 nm and 1500 nm. The thinness of the bridges beneath one micrometer results in a deep impregnation of OH by diffusion from the cavities during thermal processing, e.g. fiber drawing. Up to 1000 ppm by weight of OH had been measured in the silica web surrounding the hollow core. Obviously, the OH sources are atmospheric humidity and condensed water originating from using a hydrogen-oxygen torch at final preform fabrication. The paper shows the good agreement between OH diffusion simulation and experimental observation of OH impregnation in the hollow core web.
We demonstrate a new technique to generate a continuous-wave supercontinuum based on the stimulated Raman effect in an Yb-based ring laser configuration. Continuously pumping this ring cavity with a maximum of 19W optical power, we were able to excite up to 6 Stokes orders and achieved wavelengths up to 1600nm. Due to the feedback mechanism of the ring cavity the generated spectrum does not exhibit plain and separated stokes peaks but the fundamental Raman nature of the spectrum is altered additional nonlinear effects. This results in a dense coverage and an almost complete excitation of the wavelength range from the laser wavelength to the highest stokes wavelength and hence in a continuous-wave supercontinuum. Since the main mechanism of broadening is the Raman effect, we do not rely on anomalous dispersion and modulation instability as typically required to seed continuous wave supercontinuum generation.
Today fiber Bragg gratings are commonly used in sensing technology as well as in telecommunications. Numerous requirements must be satisfied for their application as a sensor such as the number of sensors per system, the measurement resolution and repeatability, the sensor reusability as well as the sensor costs. In addition current challenges need to be met in the near future for sensing fibers to keep and extend their marketability such as the suitability for sterilization, hydrogen darkening or the separation of strain and temperature (or pressure and temperature). In this contribution we will give an outlook about trends and future of the fiber Bragg gratings in sensing technologies. Specifically, we will discuss how the use of draw tower grating technology enables the production of tailored Bragg grating sensing fibers, and we will present a method of separating strain and temperature by the use of a single Bragg grating only, avoiding the need for additional sensors to realize the commonly applied temperature compensation.
We demonstrate three-level laser operation at 976 nm of a large-core Yb-doped aluminosilicate fiber, which is fabricated by powder-sinter technology and shows a very homogeneous refractive index profile. The investigated fiber has a core diameter of 126 μm and a numerical aperture of 0.18, well-matched to standard fiber coupled pump diodes. The core composition has been optimized to reduce photodarkening effects. Multimode and single mode operation with multiple Watts output power is presented for this fiber making it useful for the realization of high brightness fiber coupled pump sources.
We demonstrate volume Bragg gratings inscribed in S-TIH53 glass. S-TIH53 is in the proper meaning not
photosensitive; therefore we used a fs-laser system for the inscription process. The grating structure was formed in a
Talbot interferometer and was investigated with help of the external Bragg reflection method. With this method we could
measure the reflectivity profile and thereto the size of the grating. To ensure that the generated gratings are no surface or
absorption gratings the probes were investigated by a microscope and absorption measurements and heating experiments
were done.
Few-cycle pulses offer a wide range of interesting applications, for example in time-resolved studies of ultra-fast
phenomena in physics, chemistry and biology. Nonlinear spectral broadening in photonic crystal fibers (PCFs) followed
by dispersive compression allows for the generation of extremely short optical pulses. By employing this technique pulse
durations of only 5.5 fs (2.4 optical cycles) have been achieved so far. In this contribution we take advantage of SC
generation in all-normal dispersion PCF (ANDi PCF), which features only positive group-velocity dispersion across the
spectral region of interest. Spectral broadening therefore is dominated by self-phase modulation and optical wave
breaking, leading to smooth and highly coherent SC spectra. We show generation of SC spectra covering more than one
optical octave around 810 nm central wavelength. Active phase control and spectral shaping were employed to compress
the pulses to 3.64 fs (1.3 optical cycles), which is the shortest pulse duration achieved from SC compression in solid core
fibers to date. In contrast to other approaches, the presented concept delivers pulses with an excellent temporal pulse
quality and can be extended to even larger bandwidths to reach the sub-cycle regime, provided an adequate compressor
is employed.
Recently, the generation of coherent, octave-spanning, and recompressible supercontinuum (SC) light has been
demonstrated in optical fibers with all-normal group velocity dispersion (GVD) behavior by femtosecond
pumping. In the normal dispersion regime, soliton dynamics are suppressed and the SC generation process is
mainly due to self-phase modulation and optical wave breaking. This makes such white light sources suitable for
time-resolved applications.
The broadest spectra can be obtained when the pump wavelength equals the wavelength of maximum all-normal
GVD. Therefore each available pump wavelength requires a specifically designed optical fiber with suitable
GVD to unfold its full power.
We investigate the possibilities to shift the all-normal maximum dispersion wavelength in microstructured
optical fibers from the near infra red (NIR) to the ultra violet (UV). In general, a submicron guiding fiber core
surrounded by a holey region is required to overcome the material dispersion of silica.
Photonic crystal fibers (PCFs) with a hexagonal array of holes as well as suspended core fibers are simulated for
this purpose over a wide field of parameters. The PCFs are varied concerning their air hole diameter and pitch
and the suspended core fibers are varied concerning the number of supporting walls and the wall width.
We show that these two fiber types complement each other well in their possible wavelength regions for allnormal
GVD. While the PCFs are suitable for obtaining a maximum all-normal GVD in the NIR, suspended core
fibers are well applicable in the visible wavelength range.
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.
We numerically investigate supercontinuum (SC) generation in fibers with all-normal group velocity dispersion (GVD)
under femtosecond pumping, including photonic crystal fibers (PCF), optical nanofibers and suspended core PCF. It is
shown that all-normal dispersion (ANDi) fibers are ideally suited to generate extremely flat and more than octave
spanning SC spectra which are highly coherent over the entire bandwidth. Due to the suppression of soliton fission in the
normal GVD regime, the SC spectra are mainly generated by self-phase modulation and optical wave breaking
dynamics, resulting in smooth spectral profiles without significant fine structure. A single pulse is maintained in the time
domain, which can be externally compressed to the few-cycle regime. We present specific design examples of ANDi
PCF for pumping at 1080 nm and extend the concept to optical nanofibers for deep ultraviolet (UV) SC generation at
400 nm pump wavelength as well as tapered suspended core PCF for visible and near UV SC generation at 465 nm and
530 nm pump wavelength. First "proof of principle" experiments confirm the basic findings of the numerical simulations
and show the feasibility of the proposed SC generation scheme.
We report about spectral transmission behavior and influence on chromatic dispersion of index guiding microstructured
fibers (MOFs) in terms of material effects of the light propagating core, geometric parameters of the microstructured
cladding and preparation parameters. Two core compositions were investigated pure silica and silica-germania glass with
maximum 36 mol% GeO2. The MOFs with large pitch (>5 μm) were manufactured by single step technique. Small pitch
MOFs were prepared by dual step method. They show a relatively high OH absorption. The dual step prepared silicagermania
MOFs show a more than one order of magnitude higher hydroxide contamination compared to similar silica
MOFs. This result seems to be caused by the higher permeation of hydroxide groups in silica-germania glass compared
to silica. Simulations show that the red shift of the zero dispersion wavelength (ZDW) caused by high germanium doping
can be compensated by a holey cladding structure with medium up to large ratios of d/Λ.
Freestanding nanofibers with submicron diameter exhibit a strong waveguide dispersion and field localization caused by
both the extreme small core diameter compared to common optical standard fibers and the large refractive index
difference of Δn = 0.45 between the silica core and air cladding. These are promising characteristics for nonlinear
applications like e.g. supercontinuum generation that require waveguides with large nonlinear coefficients and specific
dispersion behavior in dependence on the pump wavelength. Optical nanofibers allow a shift of the zero dispersion
wavelength and in addition anomalous dispersion down to a wavelength of about 460 nm. We report on deep ultraviolet
broadband supercontinuum generation in optical nanofibers pumped with femtosecond pulses from a frequency doubled
titan-sapphire oscillator at 400nm wavelength. Numerical simulations of the generated spectra show a remarkable
broadening in the deep ultraviolet wavelength range below 250 nm after only a few millimeter of propagation distance,
while a single recompressible pulse is maintained in the time domain. The spectrum can be influenced by nanofiber
diameter, pulse duration, and pulse energy. In experiment these nanofibers are situated in an optical fiber taper
configuration where a waist of constant submicron diameter is located between two taper transitions with varying
diameter. For this reason the generated supercontinuum is not only influenced by the nanofiber diameter but by the taper
transitions.
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