High Average Power Levels Research Articles

Investigation of two-beam-pumped quasi-parametric amplification.

Multi-beam-pumped optical parametric amplification (OPA) may offer a solution for overcoming the limitations on the repetition rate and average power of ultraintense femtosecond lasers. However, multi-beam pumping scheme encounters challenges due to low conversion efficiency caused by parametric diffraction and inter-channel crosstalk, which are attributed to the presence of multi-idler-waves. This paper introduces and explores multi-beam-pumped quasi-parametric amplification (QPA) as an alternative approach to multi-beam-pumped OPA. We theoretically demonstrate that strong idler dissipation can significantly reduce inter-channel crosstalk and parametric diffraction in a two-beam-pumped QPA. These findings highlight the potential of multibeam-pumped QPA for efficiently amplifying femtosecond laser pulses to high average power levels with robustness.

Actively Q switched radially polarized Ho:YAG laser with an intra-cavity laser-written S-waveplate.

Nanosecond Q switched pulses and radial polarization are established stand-alone techniques for enhanced laser materials processing applications, but are generally challenging to achieve simultaneously at high average power levels. Here, we demonstrate a 20.6 W radially polarized Ho:YAG rod laser which has been actively Q switched in order to generate 515 μJ, 210 ns pulses at 2097 nm. By utilizing an ultra-low-loss spatially variant birefringent wave plate (S-waveplate) inside the laser cavity, the linearly polarized fundamental mode has been converted to a radially polarized donut-shaped beam with very high conversion efficiency.

50-W average power Ho:YAG SESAM-modelocked thin-disk oscillator at 2.1 µm.

Ultrafast laser systems operating with high-average power in the wavelength range from 1.9 µm to 3 µm are of interest for a wide range of applications for example in spectroscopy, material processing and as drivers for secondary sources in the XUV spectral region. In this area, laser systems based on holmium-doped gain materials directly emitting at 2.1 µm have made significant progress over the past years, however so far only very few results were demonstrated in power-scalable high-power laser geometries. In particular, the thin-disk geometry is promising for directly modelocked oscillators with high average power levels that are comparable to amplifier systems at MHz repetition rate. In this paper, we demonstrate semiconductor saturable absorber mirror (SESAM) modelocked Ho:YAG thin-disk lasers (TDLs) emitting at 2.1-µm wavelength with record-holding performance levels. In our highest average power configuration, we reach 50 W of average power, with 1.13-ps pulses, 2.11 µJ of pulse energy and ∼1.9 MW of peak power. To the best of our knowledge, this represents the highest average power, as well as the highest output pulse energy so far demonstrated from a modelocked laser in the 2-µm wavelength region. This record performance level was enabled by the recent development of high-power GaSb-based SESAMs with low loss, adapted for high intracavity power and pulse energy. We also explore the limitations in terms of reaching shorter pulse durations at high power with this gain material in the disk geometry and using SESAM modelocking, and present first steps in this direction, with the demonstration of 30 W of output power, with 692-fs pulses in another laser configuration. In the near future, with the development of a next generation of SESAM samples for this wavelength region, we believe higher pulse energy approaching the 10-µJ regime, and sub-500-fs pulses should be straightforward to reach using SESAM modelocking.

Laser Performance of Neodymium- and Erbium-Doped GYSGG Crystals

Garnet crystals possess many properties that are desirable in laser host materials, e.g., they are suitable for diode laser (LD) pumping, stable, hard, optically isotropic, and have good thermal conductivity, permitting laser operation at high average power levels. Recently, a new garnet material, GYSGG, was developed by replacing some of the yttrium ions (Y3+) with gadolinium ions (Gd3+) in YSGG, demonstrating great potential as a laser host material. GYSGG crystals doped with trivalent neodymium ion (Nd3+) and erbium ions (Er3+) were successfully grown for laser generation in the near- and mid-infrared range, with some of the laser performances reaching the level of mature laser gain media. This paper gives an overview of the achievements made in Nd3+- and Er3+-doped GYSGG lasers at different wavelength ranges. Additionally, full descriptions on Q-switching, mode-locking and wavelength-selecting methods for Nd:GYSGG, and the mechanisms of power scaling by co-doping sensitizers and deactivators in Er:GYSGG, are given. It is expected that this review will help researchers from related areas to quickly gain an understanding of these laser materials and promotes their commercialization and applications.

High power monolithic tapered ytterbium-doped fiber laser oscillator

In the power scaling of monolithic fiber lasers, the fiber nonlinear effects and transverse mode instability are main limitations. The tapered gain fiber has a longitudinally varying mode area, which has the advantage of mitigating fiber nonlinear effects. However, the transverse mode instability (TMI) was seldom reported in the tapered fiber lasers at high average power levels. In this work, we have constructed a monolithic tapered ytterbium-doped fiber laser oscillator and investigated the laser oscillator performance with respective 976 nm and 915 nm pump, especially on the aspects of the TMI. The double cladding tapered ytterbium-doped fiber has a narrow end of ~20/400 μm and a wide end of ~30/600 μm. Fiber Bragg gratings (FBG) are respectively inscribed on double cladding fibers with core/inner cladding diameter of 20/400 μm and 30/400 μm to match with the narrow and wide end of the tapered ytterbium-doped fiber. When 915 nm pump is employed, the TMI occurs at the output power of ~1350 W. The output power is further scaled to a maximum of 1720 W. The M2 factor of the output laser is ~2.1 and the full width at half maximum (FWHM) of the signal laser is ~3.6 nm. To the best of our knowledge, this is the highest average power for the tapered ytterbium-doped fiber lasers.

Vortex laser from anti-resonant ring coupled cavities.

Optical vortex Laguerre-Gaussian (LG0l) modes have wide-ranging applications due to their annular spatial form and orbital angular momentum. Their direct generation from a laser is attractive, due to the pure and high-power modes possible; however, previous demonstrations have had limited ranges of applicability. Here, we propose and implement direct LG0l vortex mode generation with an anti-resonant ring (ARR) coupled laser cavity geometry, where the gain medium inside the ARR is shared between two laser cavities. This generation uses standard wavelength-insensitive optical components, is suitable for high peak and average power levels, and could be applied to any bulk gain medium in pulsed or continuous wave regimes. This work demonstrates the technique with a diode end-pumped Nd:YVO4 gain medium. From 24 W of pump power, 8.9 W LG01 and 4.3 W LG02 modes were generated, all with high mode purity and pure handedness. The LG01 mode handedness was controlled with a new technique.

An Overview on Yb-Doped Transparent Polycrystalline Sesquioxide Laser Ceramics

This paper is devoted to summarizing and discussing the scientific progress achieved during the past decade in diode-pumped ytterbium-doped solid-state lasers. The output radiation centered on the 1- μ m region is of particular interest for the well-known applications of previously developed Nd3+ lasers. Several hosts have been studied and tested in recent years. Due to the enormous number of papers dealing with ytterbium, Yb 3+, doped hosts, we will limit our report here to materials based on transparent polycrystalline sesquioxide laser ceramics, such as Lu2O3, Sc2O3, and Y2O3 . The reason is basically due to the wider emission band of ytterbium-doped sesquioxides as compared with more commonly used hosts such as YAG. The higher thermal conductivity with respect to hosts such as CaF2 makes them very interesting for applications requiring both good thermal conductivity and a wide fluorescence emission band, as well as for short pulse generation at high average power levels. Furthermore, the replacement of trivalent Yb in sesquioxides does not require any charge compensation (as is the for instance case of the replacement of divalent Ca in CaF2), which makes their fabrication easier, in particular at high doping levels.

The Diamond Window with Boron-Doped Layers for the Output of Microwave Radiation at High Peak and Average Power Levels

We propose a novel design of the barrier window for the output of microwave radiation at high peak and average power levels. A window based on a plate of polycrystalline CVD diamond with thin (nanometer-thick) boron-doped layers with increased conductivity is considered. Such a window, which retains the low radiation loss due to the small total thickness of the conductive layers and the high thermal conductivity inherent in diamond, prevents accumulation of a static charge on its surface, on the one hand, and allows one to produce a static electric field on the surface of the doped layer, which impedes the development of a multipactor discharge, on the other hand. In this case, a high level of the power of the transmitted radiation and a large passband width are ensured by choosing the configuration of the field in the form of a traveling wave inside the window.

Sub-100-fs 187 GHz mode-locked fiber laser using stretched-soliton effects

Current pulsed fiber lasers that are capable of delivering stable sub-100-fs pulses at megahertz repetition rates require intracavity pulse energies in the nanojoule range. Scaling these lasers to gigahertz repetition rates necessitates, therefore, very high average power levels and complex cladding-pumped configurations. Here we report a type of stretched-soliton all-fiber laser that generates broadband, soliton-like pulses at 1.55 μm with intracavity pulse energies of only tens of picojoules. In the laser cavity, strong dispersion management leads to a temporal breathing ratio of ∼70, while the weak residual anomalous dispersion is perfectly balanced by the low Kerr nonlinearity, resulting in the formation of temporally stretched, hyperbolic-secant pulses. A lumped wavelength-dependent attenuator compensates for the effects of the gain filtering on the pulse spectrum, ensuring intracavity pulse self-consistency. This unique stretched-soliton mechanism, combined with a harmonic mode-locking technique based on intense optoacoustic interactions in solid-core photonic crystal fiber, yields for the first time stable gigahertz-rate, sub-100-fs, dispersive-wave-free pulse trains at moderate pump powers.

High-average-power, intense THz pulses from a LiNbO3 slab with silicon output coupler

Many applications of THz radiation require high fields and high repetition rates at the same time, implying substantial average power levels. Here, we report high-power Cherenkov-type THz generation in a LiNbO3 slab covered with a silicon prism outcoupler, a geometry in which the ratio between heat-removing surfaces and pump volume is naturally maximized for facilitating heat removal. At a conversion efficiency of 0.04 %, we achieve ~100 times more output power than before with such geometry. Although about 10 % of the 15 W pump power is converted to heat via multi-photon absorption effects, the peak crystal temperature increases by only 8 K. This result is due to the focus’ extreme aspect ratio of ~100, indicating the scalability of the approach to even higher average power levels. A line-shaped focus should be advantageous for removing heat in other optical conversions as well.

Dual-mode supply modulator for CMOS envelope tracking power amplifier integrated circuit

A dual-mode supply modulator is presented for a complementary metal-oxide semiconductor envelope tracking power amplifier (PA) integrated circuit. The proposed supply modulator, based on a hybrid buck converter, has dual Class-AB buffers and dual switches for the linear and switching amplifiers, respectively. For high average output power levels [or high-power mode (HPM)], the supply modulator is controlled to have full sizes for both the buffer and switch to supply peak envelope voltage and high output current to the PA. The supply modulator is reconfigured to have smaller sized Class-AB buffer and switch for low average output power levels (or low-power mode). The smaller sized buffer and switch are used to reduce the quiescent current of the linear amplifier and to maintain high efficiency of the switching amplifier for a lightly loaded condition. For the 16-QAM LTE uplink signal with a channel bandwidth of 5 MHz, the PA with the proposed supply modulator delivered a power-added efficiency of 46.2% at an average output power of 24 dBm for a high power mode, which shows an improvement of 8.0% compared to the stand-alone PA. For a low power mode, the PA with a reconfigured supply modulator has a power-added efficiency of 12% at an output power of 9 dBm, which is a back-off of 15 dB. This is 1.6 and 7.2% higher than that using the supply modulator for the HPM and for the stand-alone PA at the same output power, respectively. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:1338–1343, 2015

Low-Loss Transmission Lines for High-Power Terahertz Radiation.

Applications of high-power Terahertz (THz) sources require low-loss transmission lines to minimize loss, prevent overheating and preserve the purity of the transmission mode. Concepts for THz transmission lines are reviewed with special emphasis on overmoded, metallic, corrugated transmission lines. Using the fundamental HE(11) mode, these transmission lines have been successfully implemented with very low-loss at high average power levels on plasma heating experiments and THz dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) experiments. Loss in these lines occurs directly, due to ohmic loss in the fundamental mode, and indirectly, due to mode conversion into high order modes whose ohmic loss increases as the square of the mode index. An analytic expression is derived for ohmic loss in the modes of a corrugated, metallic waveguide, including loss on both the waveguide inner surfaces and grooves. Simulations of loss with the numerical code HFSS are in good agreement with the analytic expression. Experimental tests were conducted to determine the loss of the HE(11) mode in a 19 mm diameter, helically-tapped, three meter long brass waveguide with a design frequency of 330 GHz. The measured loss at 250 GHz was 0.029 ± 0.009 dB/m using a vector network analyzer approach and 0.047 ± 0.01 dB/m using a radiometer. The experimental results are in reasonable agreement with theory. These values of loss, amounting to about 1% or less per meter, are acceptable for the DNP NMR application. Loss in a practical transmission line may be much higher than the loss calculated for the HE(11) mode due to mode conversion to higher order modes caused by waveguide imperfections or miter bends.

New Light: What is Free Electron Laser FEL?

Free electrons are to be distinguished from electrons which are bound in atoms or molecules. The electrons in a Free Electron Laser (FEL) have the form of an electron beam in a vacuum, for example, like as the beam in Braun tube of TV. Electrons bound in atoms and molecules vibrate only at specific frequencies. Thus, the laser light from conventional lasers, which make use of bound electrons, appears only at these specific frequencies. On the other hand, the electrons in FEL are forced to vibrate by their passage through an alternating magnetic field. Thus, the vibration frequency can be adjusted by altering the construction of the magnetic field or by changing the speed of the electrons passing through the magnetic field. This changes the laser frequency or, equivalently, the wavelength. The broad tunability of FEL from the far infrared to the visible and beyond was the origin of the great interest in these lasers. More recently it has been recognized that free electron lasers have unique advantages for operation at high average power levels, and this has made them attractive for medical and industrial applications.

Nonlinear femtosecond pulse compression at high average power levels by use of a large-mode-area holey fiber.

We demonstrate that nonlinear fiber compression is possible at unprecedented average power levels by use of a large-mode-area holey (microstructured) fiber and a passively mode-locked thin disk Yb:YAG laser operating at 1030 nm. We broaden the optical spectrum of the 810-fs pump pulses by nonlinear propagation in the fiber and remove the resultant chirp with a dispersive prism pair to achieve 18 W of average power in 33-fs pulses with a peak power of 12 MW and a repetition rate of 34 MHz. The output beam is nearly diffraction limited and is linearly polarized.

High-performance diode-pumped Nd:YLF amplifier.

We report on the performance of a multipass diode-pumped amplifier design to provide a combination of high gain and efficiency with high stability. A simple rod-cavity design and the establishment of quasi-steady-state operation resulted in a saturated gain of over 6000 at an average output intensity during the pulse train of 7 kW/cm2. The amplifier showed an output stability of 0.2% rms in the short-term and 0.7% rms in the long-term and an output intensity insensitive to input power changes. Zernike analysis of the measurements of pump distortion showed an almost pure astigmatic phase error that can be compensated up to high average power levels.

Two-Photon near Infrared Excitation in Living Cells

Non-linear effects due to two-photon near infrared (NIR) excitation of endogenous and exogenous cellular chromophores allow novel techniques in tissue, cell and biomolecule diagnostics, as well as in intracellular micromanipulation (e.g. intracellular photochemistry). Two-photon NIR excitation may also result in cell damage effects. The high photon intensities (1024 photons cm−2 s−1) required for non-resonant two-photon excitation can be achieved by diffraction-limited focusing of continuous wave (cw) laser beams (cw microbeams) with powers in the mW range. For example, NIR traps (“laser tweezers”) used as force transducers and micromanipulation tools in cellular and molecular biology are sources of two-photon excitation. NIR traps can induce two-photon excited visible fluorescence and, in the case of <800 nm-traps, UVA-like cell damage. Multimode cw microbeams may enhance non-linear effects due to longitudinal mode-beating. To perform high scan rate two-photon fluorescence imaging, the application of ultrashort laser pulses of moderate peak power but low average power (pulsed microbeams) is required. In NIR femtosecond microscopes, non-destructive imaging of two-photon excited fluorophores in various human and culture cells was demonstrated for <2 mW average powers, <200 mW peak powers and 400 GW cm−2 intensities (700–800 nm, ∼150 fs, ∼100 MHz). However, higher average power levels may result in failed cell reproduction and cell death due to intracellular optical breakdown. In addition, destructive transient local heating and μN force generation may occur.

Birefringence compensation in polarization coupled lasers

The use of optical rotators and waveplates to compensate for thermally induced birefringence in cylindrical laser rods is reported. The use of these components allows polarization coupled lasers to operate efficiently at high average power levels. Experimentally a Nd:YAG oscillator has been operated in a Q-switched TEM(00) mode giving 300-mJ pulses and 15 W of average power.

Experience with the LAMPF Line D Fast Kicker System

In a paper presented at the 1977 Particle Accelerator Conference, R. Faulkner and R. Cooper outlined the design of the LAMPF Line D Fast Kicker System. At that time the prototype had been constructed and initial tests were complete. After the Kicker was reassembled in final operating configuration, some components were extensively modified, and considerable operational experience has since been obtained. The system has now operated in production for over 500 hours. The modifications made to the original design were few but important in terms of reaching the system design objectives. Much of the continuing component development program has centered on upgrading reliability at sustained high average power levels. The Kicker is presently capable of delivering 7-μs-long pulses at a 120-Hz repetition rate with a peak current of 5800 A. It has operated for complete LAMPF prodtuction cycles without maintenance. Continued development work has concentrated on the high power load resistor, the pulse forming network (PFN) capacitor bank, the charging inductor, and the elimination of component failure caused by high voltage/current discharges. This paper discusses Kicker problem areas and solutions.

High-temperature behavior of ZnO-based ceramic varistors

Current-voltage data for metal oxide varistors are presented in the temperature range 100–1300 °C and for current densities as high as 103 A/cm2. The current-voltage characteristic can be crudely represented at all temperatures by a breakdown line I=CVα where α≳30 (and is somewhat voltage dependent) and a temperature-dependent Ohmic leakage conduction, α=1, which is smoothly joined to the breakdown characteristic by a transition region (1<α<30) called the prebreakdown region. The Ohmic conductance is associated with a thermal excitation process over grain-boundary barriers and can be resolved into three distinct thermally activated regions which are correlated with similar features found in conductivity data for pure sintered ZnO. Low-field capacitance data on varistors show that all or part of these barriers can be attributed to depletion layers in the ZnO grains. Additional studies at very high average power levels show that the varistor characteristic can be degraded when the temperature is allowed to increase unduly due to self-heating. This degradation effects only the Ohmic and prebreakdown regions; the breakdown characteristic is unaltered. The varistor can be restored to its initial state by annealing.

Feasibility of a High Average Power Crossed Field Closing Switch

Results of an experimentalprogram to determine the feasibility of using a crossed field device as a high average power triggered closing switch are reported. The tube contained coaxial, cylindrical electrodes and was triggered by pulsing a magnetic field (<0.1 T) to a value sufficiently high to trap electrons and initiate conduction. Holdoff voltages up to 60 kV were achieved, with peak (circuit limited) currents as high as 20 kA and pulse durations in the range of 1 to 100 ?s. Ignition jitter was ~0.1 ?s, and the voltage recovery rate was 2 kV/?s after 20 kA conduction and a 50 ?s deionization time. These single and double shot data indicate that it should be possible to build such a device to operate at high average power levels for use in high power (MW) modulators.