Limited Repetition Rate Research Articles

Toward visible ultrafast imaging with a synchronously pumped switching wave Kerr frequency comb

The pursuit of real-time, high-resolution imaging at visible wavelength has long been hampered by the limitations of traditional laser sources. Existing visible ultrafast lasers often suffer from limited repetition rates, or complex pulse shaping requirements, hindering their applicability for advanced imaging techniques. This work introduces a novel ultrafast imaging technology using a 775 nm near visible Kerr frequency comb. Kerr frequency combs arise from the formation of stable localized dissipative structures in coherently driven Kerr resonators. Compared to conventional visible lasers, Kerr frequency combs offer unique advantages, such as adjustable spectral bandwidth, precise frequency control, and robust coherence. Particularly, Kerr frequency combs generated using a synchronously pumped scheme can offer flexible repetition rates from the order of hundreds of MHz to tens of GHz, overcoming the limited repetition rates of the existing visible imaging lasers. In this work, we realize a Kerr frequency comb source centred at 775 nm through switching wave generation and use this source in a proof-of-concept demonstration of ultrafast biological imaging. Experimentally, we are able to achieve a 500 MHz line-scan rate and demonstrate a new approach to high speed imaging of biological samples with unprecedented speed and versatility. We envisage our work will pave the way for exciting new discoveries in various fields, propelling us closer to a future where ultrafast dynamics can be visualized in real-time and with exquisite detail.

High repetition-rate 0.5 Hz broadband neutron source driven by the Advanced Laser Light Source

Neutron beams are an essential tool to investigate material structure and perform nondestructive analysis, as they give unique access to element composition, thus ideally complementing density analysis allowed by standard x-rays investigation. Laser-driven neutron sources, though compact and cost-effective, currently have lower average flux than conventional neutron sources, due to the limited repetition rate of the lasers used so far. However, advancements in laser technology allow nowadays to address this challenge. Here, we report results obtained at the Advanced Laser Light Source characterizing stable production of broadband (0.1–2 MeV) neutrons produced at a high repetition rate (0.5 Hz). The interaction of laser pulses of 22 fs duration and 3.2 J on-target energy with 2-μm-thick tantalum targets produced protons in the Target Normal Sheath Acceleration (TNSA) regime up to 7.3 MeV. These protons were subsequently converted into neutrons by (p,n) reactions in lithium fluoride (LiF). Activation measurements and bubble detectors were used to characterize neutron emissions, with a neutron fluence of up to ∼1.4×105 neutrons/shot/sr and energies mainly between a few hundred of kilo-electron volt and 2 MeV. The total neutron yield was ∼5×105 neutrons/shot. This paves the way for numerous applications, e.g., in homeland security, materials science, or cultural heritage.

High-energy, low-jitter, narrowband ps probe laser for kHz-rate fs/ps coherent anti-Stokes Raman scattering.

Hybrid fs/ps coherent anti-Stokes Raman scattering (CARS) thermometry often utilizes ps probe pulses derived from pulse shaping or spectrally filtering the primary laser source or by synchronization with a low repetition rate external laser. This results in limited energy, spectral resolution, and/or repetition rate of the ps probe. In this work, a master-oscillator power-amplifier (MOPA) laser was synchronized to the oscillator of a Ti:sapphire regenerative amplifier to achieve high-energy (600 µJ), narrowband (58 ps) probe pulses at kHz repetition rates. Temporal filtering allows the pulse characteristics to be adjusted for each application. At 25 Torr, relevant to high-speed flows, the kHz-rate MOPA system generated signal-to-noise ratios 3× higher in nitrogen and had improved precision relative to a 10 ps probe derived from spectral filtering and the power-amplifier. The MOPA system also enabled single-shot ro-vibrational hybrid fs/ps CARS thermometry in 650 K heated air.

Temporal feedback control of high-intensity laser pulses to optimize ultrafast heating of atomic clusters

We describe how active feedback routines can be applied at a limited repetition rate (5 Hz) to optimize high-power (>10 TW) laser interactions with clustered gases. Optimization of x-ray production from an argon cluster jet, using a genetic algorithm, approximately doubled the measured energy through temporal modification of the 150 mJ driving laser pulse. This approach achieved an increased radiation yield through exploration of a multi-dimensional parameter space, without requiring detailed a priori knowledge of the complex cluster dynamics. The optimized laser pulses exhibited a slow rising edge to the intensity profile, which enhanced the laser energy coupling into the cluster medium, compared to the optimally compressed FWHM pulse (40 fs). Our work suggests that this technique can be more widely utilized for control of intense pulsed secondary radiation from petawatt-class laser systems.

Advances in High-Power, Ultrashort Pulse DPSSL Technologies at HiLASE

The development of kW-class diode-pumped picosecond laser sources emitting at various wavelengths started at the HiLASE Center four years ago. A 500-W Perla C thin-disk laser with a diffraction limited beam and repetition rate of 50–100 kHz, a frequency conversion to mid-infrared (mid-IR), and second to fifth harmonic frequencies was demonstrated. We present an updated review on the progress in the development of compact picosecond and femtosecond high average power radiation sources covering the ultraviolet (UV) to mid-IR spectral range at the HiLASE Center. We also report on thin-disk manufacturing by atomic diffusion bonding, which is a crucial technology for future high-power laser development.

All-Optical Switching of Magnetic Tunnel Junctions with Single Subpicosecond Laser Pulses

Control of magnetism without using magnetic fields enables large-scale integration of spintronic devices for memory, computation and communication in the beyond-CMOS era. Mechanisms including spin torque transfer, spin Hall effect, and electric field or strain assisted switching have been implemented to switch magnetization in various spintronic devices. Their operation speed, however, is fundamentally limited by the spin precession time to be longer than 10-100 picoseconds. Overcoming such a speed constraint is critical for the prospective development of spintronics. Here we report the demonstration of picosecond all-optical switching of a magnetic tunnel junction (MTJ)- the building block of spintronic logic and memory -only using single telecom-band, infrared laser pulses. This first optically switchable MTJ uses ferrimagnetic GdFeCo as the free layer, and its switching is directly readout by measuring its tunneling magnetoresistance with a DR/R ratio of 0.6%. An instrument limited switching repetition rate at MHz has been demonstrated, but the fundamental limit should be higher than tens of GHz. This result represents an important step toward integrated opto-spintronic devices that combines spintronics and photonics technologies to enable ultrafast conversion between fundamental information carriers of electron spins and photons.

Electron Accelerators for Novel Cargo Inspection Methods

One of the main factors limiting the performance of conventional x-ray cargo inspection with material discrimination (MD) is the interlaced mode of system operation. Such systems use pulsed linac or betatron x-ray generators and produce alternate bremsstrahlung pulses with lower- and higher- end-point energies. Consequently, these systems provide about 50mm lower penetration than a system operated in a non-interlaced mode, have a limited range of cargo areal densities with valid MD, and cannot perform MD of objects smaller than the pulse separation. Also, the limited pulse repetition rate of x-ray generators in interlaced mode limits the radiographic image quality at nominal commercial speeds of vehicles or trains.Several new methods of cargo inspection with MD were recently introduced to address the above-mentioned limitations: dual-energy methods based on Scintillation-Cherenkov detectors [1]; multi-energy method based on intrapulse time-varying of spectral content of x-ray [1,2]; multi-energy method utilized ramping-up energy packet of short x-ray pulses [3,4]; and methods based on multi-energy betatron [5,6]. All of these methods have electron accelerators as a core element. However, the accelerator requirements and, thus, their designs, are different for each system. In this paper, we will discuss the requirements for the accelerators, provide some details about their designs, and present several novel solutions for current and future projects.

Nerve–muscle activation by rotating permanent magnet configurations

The standard method of magnetic nerve activation using pulses of high current in coils has drawbacks of high cost, high electrical power (of order 1 kW), and limited repetition rate without liquid cooling. Here we report a new technique for nerve activation using high speed rotation of permanent magnet configurations, generating a sustained sinusoidal electric field using very low power (of order 10 W). A high ratio of the electric field gradient divided by frequency is shown to be the key indicator for nerve activation at high frequencies. Activation of the cane toad sciatic nerve and attached gastrocnemius muscle was observed at frequencies as low as 180 Hz for activation of the muscle directly and 230 Hz for curved nerves, but probably not in straight sections of nerve. These results, employing the first prototype device, suggest the opportunity for a new class of small low-cost magnetic nerve and/or muscle stimulators. Conventional pulsed current systems for magnetic neurostimulation are large and expensive and have limited repetition rate because of overheating. Here we report a new technique for nerve activation, namely high-speed rotation of a configuration of permanent magnets. Analytical solutions of the cable equation are derived for the oscillating electric field generated, which has amplitude proportional to the rotation speed. The prototype device built comprised a configuration of two cylindrical magnets with antiparallel magnetisations, made to rotate by interaction between the magnets' own magnetic field and three-phase currents in coils mounted on one side of the device. The electric field in a rectangular bath placed on top of the device was both numerically evaluated and measured. The ratio of the electric field gradient on frequency was approximately 1 V m(-2) Hz(-1) near the device. An exploratory series of physiological tests was conducted on the sciatic nerve and attached gastrocnemius muscle of the cane toad (Bufo marinus). Activation was readily observed of the muscle directly, at frequencies as low as 180 Hz, and of nerves bent around insulators, at frequencies as low as 230 Hz. Nerve-muscles, with the muscle elevated to avoid its direct activation, were occasionally activated, possibly in the straight section of the nerve, but more likely in the nerve where it curved up to the muscle, at radius of curvature 10 mm or more, or at the nerve end. These positive first results suggest the opportunity for a new class of small, low-cost devices for magnetic stimulation of nerves and/or muscles.

Optimization of low-order harmonic generation by exploitation of a resistive deformable mirror

The generation of harmonics from the interaction of an ultra-intense laser pulse and a gas jet is strongly influenced by the intensity and shape of the laser beam. In this work we present a sensor-less optimization procedure of the harmonics yield based on the use of an adaptive mirror. Because of the limited repetition rate of the laser source, we used a resistive actuator-deformable mirror which allows a rapid convergence to the optimal laser beam correction without a wave-front measurement. Our approach demonstrated that this kind of deformable mirror was able to enhance the harmonic yield by a factor of two in about 40 acquisitions.

A novel solid state pulsed power module for excimer laser

For effective excimer laser pumping high voltage pulses with a pulse duration of 100 ns and energies from a fraction of joules to some 10's of joules are necessary. This energy is deposited via a gas discharge into the laser vessel. The generation of these pulses has traditionally been achieved using thyratron circuits. The limited lifetime and repetition rates of thyratron circuits do not fulfill the requirements of a modern excimer lasers. The current state of the art is to use solid state circuits with insulated gate bipolar transistor (IGBT) switches, high voltage pulse transformer, and pulse compression networks. Due to the many stages, complexity is high and efficiency is moderate. To overcome this limitation a new solid state circuit for excimer laser pumping was developed. The circuit works without magnetic pulse compression. The switch is made by a stack of "off-the-shelf" metal oxide semiconductor field effect transistors (MOSFETs) and SiC diodes. In the presentation the general design and its calculation, diode and switch qualification and test results are presented. An additional emphasis is put on the voltage sharing within the stack and the gate control.

Laser beam combiner for Thomson scattering core LIDAR

The light detection and ranging Thomson scattering (TS) diagnostic is advantageous since it only requires a single view port into the tokamak. This technique requires a short pulse laser at high energy, usually showing a limited repetition rate. Having multiple lasers will increase the repetition rate. This paper presents a scanning mirror as a laser beam combiner. Measurements of the position accuracy and jitter show that the pointing stability of the laser beam is within ±25 μrad for over tens of seconds. A control feedback loop is implemented to demonstrate the long term stability. Such a system could be applied for ITER and JET.

Advanced treatment planning methods for efficient radiation therapy with laser accelerated proton and ion beams

Laser plasma acceleration can potentially replace large and expensive cyclotrons or synchrotrons for radiotherapy with protons and ions. On the way toward a clinical implementation, various challenges such as the maximum obtainable energy still remain to be solved. In any case, laser accelerated particles exhibit differences compared to particles from conventional accelerators. They typically have a wide energy spread and the beam is extremely pulsed (i.e., quantized) due to the pulsed nature of the employed lasers. The energy spread leads to depth dose curves that do not show a pristine Bragg peak but a wide high dose area, making precise radiotherapy impossible without an additional energy selection system. Problems with the beam quantization include the limited repetition rate and the number of accelerated particles per laser shot. This number might be too low, which requires a high repetition rate, or it might be too high, which requires an additional fluence selection system to reduce the number of particles. Trying to use laser accelerated particles in a conventional way such as spot scanning leads to long treatment times and a high amount of secondary radiation produced when blocking unwanted particles. The authors present methods of beam delivery and treatment planning that are specifically adapted to laser accelerated particles. In general, it is not necessary to fully utilize the energy selection system to create monoenergetic beams for the whole treatment plan. Instead, within wide parts of the target volume, beams with broader energy spectra can be used to simultaneously cover multiple axially adjacent spots of a conventional dose delivery grid as applied in intensity modulated particle therapy. If one laser shot produces too many particles, they can be distributed over a wider area with the help of a scattering foil and a multileaf collimator to cover multiple lateral spot positions at the same time. These methods are called axial and lateral clustering and reduce the number of particles that have to be blocked in the beam delivery system. Furthermore, the optimization routine can be adjusted to reduce the number of dose spots and laser shots. The authors implemented these methods into a research treatment planning system for laser accelerated particles. The authors' proposed methods can decrease the amount of secondary radiation produced when blocking particles with wrong energies or when reducing the total number of particles from one laser shot. Additionally, caused by the efficient use of the beam, the treatment time is reduced considerably. Both improvements can be achieved without extensively changing the quality of the treatment plan since conventional intensity modulated particle therapy usually includes a certain amount of unused degrees of freedom which can be used to adapt to laser specific properties. The advanced beam delivery and treatment planning methods reduce the need to have a perfect laser-based accelerator reproducing the properties of conventional accelerators that might not be possible without increasing treatment time and secondary radiation to the patient. The authors show how some of the differences to conventional beams can be overcome and efficiently used for radiation treatment.

Comparison of shutter glasses and mirror stereoscope for measuring dynamic and static vergence

Vergence eye movement recordings in response to disparity step stimuli require to present different stimuli to the two eyes. The traditional method is a mirror stereoscope. Shutter glasses are more convenient, but have disadvantages as limited repetition rate, residual cross task, and reduced luminance. Therefore, we compared both techniques measuring (1) dynamic disparity step responses for stimuli of 1 and 3 deg and (2) fixation disparity, the static vergence error. Shutter glasses and mirror stereoscope gave very similar dynamic responses with correlations of about 0.95 for the objectively measured vergence velocity and for the response amplitude reached 400 ms after the step stimulus (measured objectively with eye movement recordings and subjectively with dichoptic nonius lines). Both techniques also provided similar amounts of fixation disparity, tested with dichoptic nonius lines.

Generation of variable mixed vortex fields by a single static hologram

Recently, a transfer of information based on application of mixed vortex fields has been proposed and experimentally verified. A dynamical creation of vortex superpositions carrying an actual information code has been realized by the spatial light modulator with a limited repetition rate. In this paper, the optical set-up enabling generation of variable optical vortex superpositions by a single static hologram is proposed and examined. The actual information codes are created by a standard dynamical modulation of an array of point sources illuminating the static optical system.

Narrow line high power picosecond pulse generation in a multicontact distributed feedback laser using modified Q switching

High power bandwidth-limited picosecond pulses with peak powers in excess of 200 mW have been generated using multicontact distributed feedback laser diodes for the first time. The pulses have widths typically less than 10 ps, time-bandwidth products of as little as 0.24, and can be generated on demand at generator limited repetition rates of up to 140 MHz.

Q-switching of the CO<inf>2</inf>laser

The laser pulses obtainable from a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</tex> -switched CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> laser are calculated and compared with the results of a number of different techniques of performing the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</tex> -switching. The continuously operating laser is considered first. The transition rates between the molecular vibrational states and their occupations are derived from the measured CW power. The laser tube was 1.9 meters long, had a diameter of 2.4 cm, and used flowing CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> -N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> -He gas. For rapid <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</tex> -switching, maximum pulses of 4.5 mJ energy and 85 ns half width are predicted. Such pulses were observed with a rotating mirror <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</tex> -switch. However, that technique has a limited pulse repetition rate and experiments on closely spaced pulses are difficult to interpret. A more flexible technique, which allows a much greater variation in the experimental parameters, is the use of a fast shutter to interrupt the laser beam in the cavity. While this switch is somewhat slower than the rotating mirror it does produce pulses of the same energy at repetition rates up to 5000 per second, and smaller pulses at any desired higher rate. From these measurements the upper and lower laser level lifetimes are deduced. They are found to agree well with the values obtained from the CW measurements.