High power laser weapons have made impressive technical progress and have demonstrated their capabilities in shooting down numerous targets in live demos. National demonstrator systems on military platforms are now in preparation, for instance the integration of a 20 kW laser effector on a maritime platform. The core component of a laser weapon system is the militarized high energy laser (HEL). In this contribution the development of a 20 kW HEL for defence application will be presented. The laser concept is based on robust, compact and efficient high-power single-mode fiber laser modules. Power scaling to 20 kW is achieved with spectral beam combining. System architecture and laser data will be discussed.
With their advantages like good beam quality, easy thermal management, high robustness and compact size, fiber lasers are one of the most promising solid state laser concepts for high power scaling with excellent beam quality. One issue of further power scaling is the reduction of nonlinear effects, especially Raman scattering, which consequently led to increased mode field areas. However, for large mode area fibers, new challenges, namely transversal mode instabilities (TMI) have to be taken into account. Beside our investigations in the power scaling of ytterbium doped fiber amplifiers up to 4.4kW output power, we present our investigations of the TMI threshold in dependence on bend diameter and absorption length of a well-known, commercial fiber. Within this scope, we used a 13m piece of the fiber and gradually reduced the bend diameter from 60cm slightly below 14cm within a pump wavelength of 976nm. Furthermore, we increased the fiber length to 30 m, presuming the bend diameter of 14 cm and all experimental conditions. However, in a next step, we detuned the pump wavelength up to 980 nm in order to increase the pump absorption length As a result, we achieved 2.9kW of single mode output at a bend diameter of 14cm. The 4.4kW result was obtained with a separately manufactured low-NA fiber, allowing for a slope efficiency of 90% with regards to the absorbed pump light and an extremely temporal stability.
The average output power of fiber lasers have been scaled deep into the kW regime within the recent years. However a further scaling is limited due to nonlinear effects like stimulated Raman scattering (SRS). Using the special characteristics of femtosecond laser pulse written transmission fiber gratings, it is possible to realize a notch filter that mitigates efficiently this negative effect by coupling the Raman wavelength from the core into the cladding of the fiber. To the best of our knowledge, we realized for the first time highly efficient gratings in large mode area (LMA) fibers with cladding diameters up to 400 μm. The resonances show strong attenuation at design wavelength and simultaneously low out of band losses. A high power fiber amplifier with an implemented passive fiber grating is shown and its performance is carefully investigated.
We present investigations on the seed source dependence of stimulated Raman scattering (SRS) created in a high power fiber amplifier. It is shown that fiber oscillators are much worse in terms of SRS than other seed sources. The longitudinal mode composition was found to be of less importance. We reinforce the experimental observations by a numerical investigation, which shows that temporal power variations on the ps-scale and their propagation along the fiber are crucial for the SRS creation in high-power fiber systems, extending the well-known but simplified SRS threshold description.
The amount of cladding light is important to ensure longevity of high power fiber components. However, it is usually measured either by adding a cladding light stripper (and thus permanently modifying the fiber) or by using a pinhole to only transmit the core light (ignoring that there may be cladding mode content in the core area). We present a novel noninvasive method to measure the cladding light content in double-clad fibers based on extrapolation from a cladding region of constant average intensity. The method can be extended to general multi-layer radially symmetric fibers, e.g. to evaluate light content in refractive index pedestal structures.
To effectively remove cladding light in high power systems, cladding light strippers are used. We show that the stripping efficiency can be significantly improved by bending the fiber in such a device and present respective experimental data. Measurements were performed with respect to the numerical aperture as well, showing the dependency of the CLS efficiency on the NA of the cladding light and implying that efficiency data cannot reliably be given for a certain fiber in general without regard to the properties of the guided light.
Proc. SPIE. 9728, Fiber Lasers XIII: Technology, Systems, and Applications
KEYWORDS: Fiber amplifiers, Optical amplifiers, Backscatter, Calibration, Fiber lasers, Reflectometry, High power fiber lasers, High power fiber amplifiers, Thulium, Fiber coatings, Temperature metrology, Absorption
We present measurements of the temperature increase inside the active fiber of a thulium fiber amplifier during high power operation. At a pump power of over 100 W at a wavelength of 793 nm, we measure the core temperature distribution along the first section of a large mode area (LMA) highly thulium doped active fiber by use of an optical backscatter reflectometer. A mode field adaptor is used to maintain single mode operation in the LMA fiber. An increase in temperature of over 100 K can be observed in spite of conductive cooling, located at the pumped fiber end and jeopardizing the fiber coating. The recoated splice can be clearly identified as the hottest fiber region. This allows us to estimate the maximum thermally acceptable pump power for this amplifier. We also observe that the temperature can be decreased by increasing the seed power, which is in agreement with theoretical predictions on the increase of cross relaxation efficiency by depletion of the upper laser level. This underlines the role of power scaling of the respective seed power of a thulium amplifier stage as a means of thermal management.
In this paper the threshold for Stimulated Raman scattering (SRS) is analyzed experimentally and theoretically for monolithic LMA cq kW fiber oscillators. Four oscillators with different spectral widths of the low reflecting (LR) Fiber Bragg Gratings (FBG) (0.04 nm, 0.5 nm, 1.5 nm (FWHM) and without LR grating) were characterized. Experimental it was found that threshold of SRS depends on the spectral width of the out coupling FBGs, which is not yet understood completely. Attempts to describe such lasers by simulations are based on nonlinear Schrödinger equation supporting spectral broadening of cw-fiber laser, rate equation gain as well as broadband Raman gain. The experimental results and the simulations were compared and discussed.
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.
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.
In this work we report on what we believe is the highest pulse energy obtained from an effectively single-mode Qswitched
fiber laser system to date. This result has been obtained using rare-earth-doped double-clad large-pitch fibers
which are able to delocalize the higher-order transverse modes from the active region of the core. We have built an
actively Q-switched oscillator consisting of a 1.3m long large-pitch fiber with a mode field diameter of 50μm. This
oscillator delivered pulses with 500μJ energy at a repetition rate of 5kHz. These seed pulses have been amplified in two
fiber amplifiers which were pumped at 976nm. The pre-amplifier employs a 1.2m long large-pitch fiber with a mode
field diameter of 80μm and delivered 3.5mJ pulse energy. Then, a 135μm core diameter large-pitch fiber was used as
booster amplifier. Under high power operation the mode field diameter of this fiber is ~100μm. The setup can deliver
average powers of up to 130W, resulting in 26mJ pulses with pulse durations of ~50ns. Additionally, since this is an
effective single-mode laser system, the beam quality is close to the diffraction-limit.
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.
Lasers for marking, direct application laser systems as well as high power solid state lasers require highly reliable, high
efficient and low cost laser diodes. Especially fiber lasers and direct diode systems have additionally the need for high
brightness. For a very long time either single emitter solutions with low brightness and costs or beam shaped bar
solutions with high brightness and high costs served those needs. Since roughly 2 years multiple single emitter solution
are more and more penetrating the market showing a high potential for serving all needs of a broad customer base.
Based on the 50W product introduced by the middle of 2009 we would like to show the design which is based on
qualified and highly stable single emitters.
We present record-breaking experimental data on high power transmission through novel 7X1 and 19X1 multimode
combiners based on Photonic Crystal Fiber technology. Both combiners are monolithic, have losses of ~0.2 dB, show
very high thermal robustness and can handle record high optical powers. We have transmitted 100 W through the 7X1
and 310 W through the 19X1 combiner without evidence of any degradation or critical heating. The powers were limited
only by available pump power.
The combiners are based on Air-clad technology, where a ring of air-holes running along the length of the device provide
guiding for the light. This Air-clad offers three major advantages for the device: 1: It is well suited for high optical
powers as no polymer coating gets into contact with the light; 2: it is much easier to package as mechanical contact can
be made anywhere on the device without risk of optical performance penalty; 3: the Numerical Aperture of the light can
be increased beyond the limits imposed by polymer coatings.
The presented pump combiners are especially well suited for high power fiber lasers, since such combiners can be
spliced directly onto the active fiber, thereby enabling a robust, stabile laser solution with excellent efficiency and beam
The different concepts of combining fiber lasers for power-scaling are discussed. We report on three combined fibers with an output power of 100 W. Several proposals are made for further power scaling and the capacitance of a grating is tested in a simulation-experiment.
A Q-switched all-fiber laser application based on a novel micro-optical waveguide (MOW) on micro-actuating platform (MAP) light modulator is presented. A fused biconical taper (FBT) coupler acts as MOW, mounted on an electromechanical system, MAP, where an axial stress over the waist of FBT coupler is precisely controlled. The axial stress induced refractive index changes caused the coupling efficiency to result in modulation of optical power. The light modulator was implemented in a laser cavity as a Q-switching element. Q-switching of Yb3+-doped fiber laser was successfully achieved with the peak power of 192mW at 4.1W pump power and 699mW at 5.2W at the repetition rate of 18.6kHz. Further optimization of switching speed and modulation depth could improve the pulse extraction efficiency and the proposed structure can be readily applied in all-fiber Q-switching laser systems for marking applications.
The fiber based generation of nearly transform-limited 10-ps pulses with 200 kW peak power (97 W average power) based on SPM-induced spectral compression is reported. Efficient second harmonic generation applying this source is also discussed.
In the last years a dramatic increase of the output power of rare-earth-doped fiber lasers and amplifiers with diffraction limited beam quality has been observed. These demonstrates impressively that fiber lasers and amplifiers are an attractive and power scalable solid-state laser concept. The main limiting factors for the laser output power are the damage of the fiber ends, heating of the fiber due to the quantum defect and nonlinear effects. To overcome these problems, an increasing of the core diameter and keeping the fiber single mode, by using solid core step-index large-mode-area fibers, allow the power scaling beyond 1 kW at diffraction limited beam quality. A further scaling is possible by using novel highly doped air-clad photonic crystal fibers with increased mode field diameters of the active core. This type of fibers has several new preferable features. In our contribution we will discuss the advantages of microstructured fibers to reduce nonlinear effects inside the fiber and the possibility to scale the output power of fiber lasers and amplifiers with excellent beam quality. We also show experiments with pulsed fiber amplifier systems using these microstructured large mode area fibers.
Fiber lasers are pumped by fibercoupled, multimode single chip devices at 915nm. That’s what everybody assumes when asked for the type of fiber laser pumps and it was like this for many years.
Coming up as an amplifier for telecom applications, the amount of pump power needed was in the range of several watts. Highest pump powers for a limited market entered the ten watts range. This is a range of power that can be covered by highly reliable multimode chips, that have to survive up to 25 years, e.g. in submarine applications. With fiber lasers entering the power range and the application fields of rod and thin disc lasers, the amount of pump power needed raised into the area of several hundred watts. In this area of pump power, usually bar based pumps are used. This is due to the much higher cost pressure of the industrial customers compared to telecom customers. We expect more then 70% of all industrial systems to be pumped by diode laser bars. Predictions that bar based pumps survive for just a thousand hours in cw-operation and fractions of this if pulsed are wrong. Bar based pumps have to perform on full power for 10.000h on Micro channel heat sinks and 20.000h on passive heatsinks in industrial applications, and they do.
We will show a variety of data, “real” long time tests and statistics from the JENOPTIK Laserdiode as well as data of thousands of bars in the field, showing that bar based pumps are not just well suitable for industrial applications on high power levels, but even showing benefits compared to chip based pumps. And it’s reasonable, that the same objectives of cost effectiveness, power and lifetime apply as well to thin disc, rod and slab lasers as to fiber lasers. Due to the pumping of fiber lasers, examples will be shown, how to utilize bars for high brightness fiber coupling. In this area, the automation is on its way to reduce the costs on the fibercoupling, similar to what had been done in the single chip business. All these efforts are part of the JENOPTIK Laserdiode’s LongLifeTechnologie.
A new type of multi-clad rare-earth doped silica fiber was designed, prepared and tested for the power scaling of high power fiber lasers in the 1 .1 tm wavelength region. By means of a dedicated laboratory setup a maximum output power of more than 1 .300 watts with excellent spectral and beam behavior was achieved. The fundamental investigation of the energy transfer processes and of the fluorescence lifetimes of different Nd:Yb co-doped has been studied.
Such fiber-lasers were tested in the laboratory at several materials (plastics, metals, glass) in the fields of material processing and micro-marking, respectively.
Experimental results based on rare-earth-doped fibers have impressively shown that fiber lasers and amplifiers are an attractive and power scalable solid-state laser concept. Based on ytterbium-doped large-mode-area (LMA) double-clad fibers in the continuous regime output powers approaching the kW-range with diffraction limited beam quality have been shown. Average output powers in the order of 100 W even for nanosecond fiber amplifiers have been demonstrated in the pulsed regime. Further power scaling is limited by nonlinear effects, thermo -optical problems or amplified spontaneous emission. In our contribution we discuss power scaling of fiber lasers and amplifiers in the multi kW-range with excellent beam quality based on rare-earth-doped fibers.
Experimental results based on rare-earth-doped fibers have impressively shown that fiber lasers and amplifiers are an attractive and power scalable solid-state laser concept. Based on ytterbium-doped large-mode-area double-clad fibers, in the continuous regime, output powers approaching the kW-range with diffraction limited beam quality have been shown. Average output powers in the order of 100 W have been demonstrated in the pulsed regime even for femtosecond fiber lasers. Further power scaling is limited by the end facets damage, thermo-optical problems or nonlinear effects. To overcome these restrictions microstructured fibers with several new preferable features can be used. In our contribution we will discuss power scaling of fiber lasers and amplifiers in the multi kW-range with excellent beam quality based on rare-earth-doped photonic crystal fibers.
For emerging real world applications the availability of high repetition rate and high energy ultrashort pulse laser systems is of significant importance. Ytterbium-doped fiber laser systems have established themselves as a very attractive gain medium for pulsed amplification. We discuss the feasibility of high average power (>100 W) and high energy (~100 μJ) femtosecond fiber CPA systems. Novel fiber geometries based on microstructured large-core air-clad fibers are introduced, which allow for a significant performance scaling. Furthermore, an all-fiber CPA system including an air-core photonic band-gap fiber compressor is presented. This approach opens the avenue to a completely fiber integrated high performance short pulse laser system.
Most recently the output power of fiber lasers with diffraction limited beam quality has been significantly increased. Further power scaling is usually limited by damage of the fiber end facets, thermo-optical problems or nonlinear effects. Microstructuring the fiber adds several preferable features to the fiber to overcome these restrictions. We review the advantages of rare-earth-doped photonic crystal fibers for power scaling of fiber lasers to the multi kW range with excellent beam quality.
We review our recent work on fiber based laser systems in the continuous wave and nanosecond pulse regime. A significant power and energy scaling was possible by applying low-numerical aperture large-mode-area ytterbium-doped double-clad fibers which emit an excellent beam quality and possess a reduced nonlinearity due to a mode field diameter of >20 μm. We report on a continuous wave fiber laser with an output power approaching 500 W from a single fiber and the amplification of ultrastable, narrow linewidth single-frequency radiation to the 100 W level with diffraction-limited beam quality without the limitation of nonlinear effects. Furthermore the amplification of nanosecond pulses to millijoule pulse energy and average powers up to 100 W is demonstrated.
Fiber laser systems offer unique properties for the amplification of ultrashort pulses to high powers. Two approaches are discussed, the amplification of linearly chirped parabolic pulses and a fiber based chirped pulse amplification system. Using the first method, we succeeded to generate 17-W average power of linearly chirped parabolic pulses at 75 MHz repetition rate and diffraction-limited beam quality in a large-mode-area ytterbium-doped fiber amplifier. The recompression of these pulses with an efficiency of 60% resulted in 80-fs pulses with a peak power of 1.7 MW. Furthermore, we report on a diode-pumped ytterbium-doped double-clad fiber based chirped pulse amplification system delivering 350-fs pulses, at 1060 nm wavelength, 75 MHz repetition rate and up to 60 W average power, corresponding to a peak power of 2.3 MW. Key element is a diffraction grating compressor consisting of highly efficient transmission gratings in fused silica allowing the recompression at this high power. Power scaling to the >100 W level is discussed.