The development of highly customized technical devices is a decisive feature of technically complex setups, as frequently observed in quantum experiments. This paper describes the development and realization of an Yb-doped all-fiber amplifier system designed for such a special application, more specifically, an on-demand single-photon source based on four-wave mixing with rubidium Rydberg atoms. The laser is capable of generating bandwidth-limited configurable nanosecond pulses up to peak powers of >100 W and with pulse repetition frequencies (PRF) between 50 Hz and 1 MHz at selectable wavelengths (1008-1024 nm). Especially the amplification of the 1010 nm reference seed at the lower edge of the amplification range for Yb-based fibers is challenging and tends to produce amplified spontaneous emission (ASE) at higher wavelengths. To achieve high ASE suppression, particularly at low pulse repetition frequencies, two acousto-optical modulators (AOM) are utilized both for pulse picking and for temporal filtering. The synchronization between pulse repetition frequency and AOM driver signal allows pulse amplitude fluctuations to be kept below 1%, while ASE is suppressed by at least 85 dB (PRF = 1 MHz) and 65 dB (PRF = 1 kHz).

Laser cooling of a solid is achieved when a coherent laser illuminates the material in the red tail of its absorption spectrum, and the heat is carried out by anti-Stokes fluorescence of the blue-shifted photons. Solid-state laser cooling has been successfully demonstrated in several materials, including rare-earth-doped crystals and glasses. Silica glass, being the most widely used optical material, has so far evaded all laser cooling attempts. In addition to its fundamental importance, many potential applications can be conceived for anti-Stokes fluorescence cooling of silica. These potential applications range from the substrate cooling of optical circuits for quantum information processing and cryogenic cooling of mirrors in high-sensitivity interferometers for gravitational wave detection to the heating reduction in high-power fiber lasers and amplifiers. Here we report the net cooling of high-purity Yb-doped silica glass samples that are primarily developed for high-power fiber laser applications, where special care has been taken in the fabrication process to reduce their impurities and lower their parasitic background loss. The non-radiative decay rate of the excited state in Yb ions is very small in these glasses due to the low level of impurities, resulting in near-unity quantum efficiency. We report the measurement of the cooling efficiency as a function of the laser wavelength, from which the quantum efficiency of the silica glass is calculated.

We present modal content measurements (S2) of two different negative curvature hollow-core photonic crystal fibers: a kagome fiber and an ice cream cone fiber. Their sensitivity towards mode matching, bending and polarization is analyzed. For the kagome fiber, a higher order mode suppression of 17dB under optimal conditions was achieved, and for the ice cream cone fiber there was a suppression of up to 42dB. Polarization turned out to be a critical parameter for good higher order mode suppression in both fibers.

High-power fiber lasers have reached kW power levels. The most important non-linear process limiting power scaling of industrial fiber lasers is stimulated Raman scattering. Long period gratings (LPGs) couple forward propagating core light to forward propagating cladding light and are well suited as a filter for the unwanted Raman scattering. In this paper we show for the first time of our knowledge the inscription of LPGs in large-mode-area (LMA) fibers with ultra-short laser pulses. We investigate the influence of different inscription parameters with a 3D, spatially resolved measurement of the induced index change. We present results from gratings with an attenuation of 8.5 dB at the desired wavelength with a small out-of-band loss of 1 dB.

The generation of high power in active fiber application and the transmission of high laser power via fiber cables both require protection from misdirected laser light. The following paper presents a new approach to removing this unwanted part of light. The deposition of fused silica material on the fiber cladding applied with CO2 laser processes constitutes a robust cladding light stripper suitable for high power levels. The CO2 laser processes are easy to apply, obviate the need for any dangerous liquids and promise greater mechanical stability in handling and assembly.

Scaling of the power yield of offshore wind farms relies on the capacity of the individual wind turbines. This results in a trend to very large rotor diameters, which are difficult to control. It is crucial to monitor the inhomogeneous wind field in front of the wind turbines at different distances to ensure reliable operation and a long lifetime at high output levels. In this contribution, we demonstrate an all-fiber ns-pulsed fiber amplifier based on cost-efficient commercially available components. The amplifier is a suitable source for coherent Doppler lidar pulses making a predictive control of the turbine operation feasible.

We present a selective mode filter inscribed with ultrashort pulses directly into a few mode large mode area (LMA) fiber. The mode filter consists of two refractive index modifications alongside the fiber core in the cladding. The refractive index modifications, which were of approximately the same order of magnitude as the refractive index difference between core and cladding have been inscribed by nonlinear absorption of femtosecond laser pulses (800 nm wavelength, 120 fs pulse duration). If light is guided in the core, it will interact with the inscribed modifications causing modes to be coupled out of the core. In order to characterize the mode filter, we used a femtosecond inscribed fiber Bragg grating (FBG), which acts as a wavelength and therefore mode selective element in the LMA fiber. Since each mode has different Bragg reflection wavelengths, an FBG in a multimode fiber will exhibit multiple Bragg reflection peaks. In our experiments, we first inscribed the FBG using the phase mask scanning technique. Then the mode filter was inscribed. The reflection spectrum of the FBG was measured in situ during the inscription process using a supercontinuum source. The reflectivities of the LP01 and LP11 modes show a dependency on the length of the mode filter. Two stages of the filter were obtained: one, in which the LP11 mode was reduced by 60% and one where the LP01 mode was reduced by 80%. The other mode respectively showed almost no losses. In conclusion, we could selectively filter either the fundamental or higher order modes.

Fiber lasers have reached kW levels of output power. To achieve this level it is necessary to use reliable high-power components that sustain these power levels. Double-clad fibers (DCFs) are often used in high-power fiber lasers. Cladding-light strippers (CLSs) are used to remove unwanted light from the inner cladding of the DCF. This unwanted light consists of residual pump light or signal light that leaked into the cladding, thus requiring that the CLS removes both high-NA (>0.4) and low-NA (<0.1) light. Often high-index polymers are used to remove the unwanted light from DCFs(1,2,3). Because the CLS has to be able to withstand several 100W and most polymers are not capable of exceeding temperatures more than 200 degrees C, we investigated a CLS without polymers, based on an etching process. We present results from a CLS that was tested up to 500W of stripped power. We determined the angle dependency of the stripping efficiency by launching both high- and low-NA light into the fiber and evaluating the NA attenuation. Furthermore, we measured the dependency of the stripping efficiency on the length of the etched area and the etching time. With optimized parameters an attenuation of more than 20 dB when launching high-NA light and 6 dB with low-NA light was achieved. The CLS did not show any degradation in terms of attenuation or thermal behavior in a six-hour stability test.

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text A. Liem, E. Freier, C. Matzdorf, V. Reichel, T. Schreiber, R. Eberhardt, and A. Tünnermann, "Experimental analysis of the influence of the spectral width of out-coupling Fiber Bragg Gratings to the amount of Stimulated Raman Scattering in a cw kW fiber oscillator," in Advanced Solid-State Lasers Congress, G. Huber and P. Moulton, eds., OSA Technical Digest (online) (Optica Publishing Group, 2013), paper JTh2A.32. Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

We investigate the influence of seed polarization on nonlinear effects in a high power fiber amplifier for different orientations of the linear seed polarization and for different ellipticities of the seed polarization ( linear, elliptic, circular polarized). We show that it was possible to considerably reduce the power of the Raman scattered light. Maximum reduction to around 50% could be achieved by changing the seed polarization from linear to circular. Furthermore, we demonstrate that not only the threshold of nonlinear effects could be influenced by changing the orientation of the linear seed polarization as only parameter but even the limiting effect could be changed: For all orientations of the linear seed polarization Raman scattering was the dominant nonlinear effect except for linear polarization along the slow fiber axis of the slightly birefringent amplifier fiber, where also modulation instability was observed. From our results we estimate the importance of the polarization state as further parameter to increase the nonlinear threshold of high power fiber amplifier systems.

The application of photonic crystal fibers (PCF), especially in high power fiber laser systems, requires special preparation technologies with some significant differences compared to standard fibers. Features, like air-clad structures, highly rare-earth doped cores with low NA and stress applying parts of the PCFs, require additional steps in fiber preparation and innovative splicing technologies to gain optical properties. Here we discuss a contamination- free carbon dioxide laser splicing device, which is used for defined air-clad collapsing and end cap splicing to get a stable and sealed fiber end face with preserved high beam quality and additional functionality. The special design of the computer-controlled laser splicing process provides a versatile tool with high reproducibility for joining different geometries with an adjustable well-balanced heat distribution. A wide range of PCFs with different diameters, air-clad structures and doped materials up to ~2 mm have been spliced. For selected PCF-end cap splices cleave or polishing requirements as well as results on beam quality, tensile strength and further splicing features are presented.

We present the characterization of narrow-linewidth ASE source, which has been amplified to 1.1 kW using a RMO-fiber design. SBS is known as the main limiting effect for fiber amplified single-frequency and narrow-band signals, respectively.

The major challenge in the development of monolithic kW class CW fiber lasers is the efficient conversion of pump photons into a high brightness laser beam under the constraints of heat management, long term stability and nonlinearities. This article reviews the interaction of some fiber related aspects as e.g. fiber core composition, photodarkening and modality, as well as their influence on system complexity and power scalability. Recent work on active fibers, pump couplers, mode field adaptors and other fiber-optic components will be presented.

We report on the high power amplification of narrow linewidth laser radiation with close to diffraction limited beam quality using a large mode area photonic crystal fiber amplifier. The observation of threshold-like higher order mode amplification by transverse spatial-hole burning at the highest power level is reported. The measured M(2) stays below 1.3 but increases at the critical power level, where the fundamental mode turns into the next higher order mode. At the maximum power of 1.2 kW a linewidth of <80 pm limited by self-phase modulation is obtained.

We report on a novel concept for monolithic pump combining technology to integrate efficiently multi pump fiber channel into a double clad ytterbium doped fiber. The proposed structure consists of a dichromatically coated planar convex lens spliced to an Ytterbium-doped double-clad photonic crystal fiber surrounded by multiple pump fibers. The lens is also used as a protecting end cap where the laser beam expands before exiting the surface. The pump fibers are also attached in this lens circularly surrounding fibers. The lens images these pump fibers end facets into the pump core, where the lens surface is coated by a dichroic mirror (reflective for 980 nm, transmissive for > 1030 nm). The all-glass structure, assembled by laser splicing, makes the system stable, efficient and suitable for high power operation. We selected 5 channels as testing channels among 14 pump channels (200 mu m, NA=0.12) in order to confirm reliability of the system. The coupled pump power efficiency into the 500 mu m core with NA=0.5 was over 80% and typical slope efficiency of the laser output is over 70%. Theoretical analysis was discussed in order to get optimized parameters and scaling this type of coupler to higher average powers is considered. With the monolithic pump combining technologies, we confirmed that the proposed device has a potential application not only in kW range high power fiber lasers but also compact photonic devices.

Many dynamical phenomena in nature as well as in technological applications involve nonlinear behaviour as an essential ingredient. This fact is reflected by recent developments in time series analysis. Specifically, I will repor t on one example of a novel method that can take nonlinearity into account without assuming determinism. Qualitative as well as quantitative insights can be ga in d on the structure of a system from an understanding of the information flow between components.

We report on the fiber laser based generation of 45 W average power of 60-fs pulses using nonlinear spectral broadening in a large-mode-area photonic crystal fiber followed by compression with chirped mirrors.

The performance of high average power and high energy femtosecond fiber laser systems is discussed. Remarkable evolutions in fiber technology made it possible to overcome restrictions due to nonlinear pulse distortions in the amplification fiber and revealed the full potential of rare-earth-doped fibers as a power-scalable solid-state laser concept in the short pulse regime. State-of-the-art femtosecond fiber lasers in our labs deliver average powers well above 100 W and pulse energies of several 100 mu J in the 1 mu m wavelength region. This performance, in particular the significantly higher repetition rate compared to conventional femtosecond lasers, allows for unique approaches in several application fields. Beside the fiber designs, the setup, performance and limitations of these systems we will discuss selected applications.

A compact mode-locked fiber laser delivering 21-W of linear polarized average power at 17 MHz repetition rate and 240-fs pulse duration is reported. Using ytterbium-doped polarization-maintaining and single-polarization fibers ensures environmental stability of the short-pulse laser.