A fiber-optic RF distribution system has been developed for synchronizing lasers and RF plants in short pulse FELs.
Typical requirements are 50-100fs rms over time periods from 1ms to several hours. Our system amplitude modulates a
CW laser signal, senses fiber length using an interferometer, and feed-forward corrects the RF phase digitally at the
receiver. We demonstrate less than 15fs rms error over 12 hours, between two independent channels with a fiber path
length difference of 200m and transmitting S-band RF. The system is constructed using standard telecommunications
components, and uses regular telecom fiber.
We describe a 60W, 70fs, 20kHz Ti:sapphire CPA laser system using cryogenically-cooled amplifiers, currently
operating at the Advanced Light Source at LBNL. The system consists of an oscillator, a 20 kHz regenerative preamplifier,
and two power amplifiers to produce two output beams, each at 30W. Each power amp can be pumped by two
90 Watt, 10 kHz, diode-pumped, doubled YLF lasers simultaneously (for 10 kHz) or interleaved in time (for 20 kHz).
The regen is pumped at 20 kHz and 60W, producing 8W output which is split between the power amps. To maintain the
crystals near the thermal conductivity peak at ~50°K, we used 300 Watt cryorefrigerators mechanically decoupled from
the optical table. Pulses are compressed in a quartz transmission grating compressor, to minimize thermal distortions of
the phase front typical of gold coated gratings at high power density. Transmission through the compressor is >80%,
using a single 100 x 100mm grating. One of the 30W output beams is used to produce 70fs electron bunches in the
synchrotron light source. The other is delayed by 300ns in a 12-pass Herriot cell before amplification, to be
synchronized with the short light pulse from the synchrotron.
We describe the design concepts for a potential future source of femtosecond x-ray pulses based on synchrotron radiation production in a recirculating electron linac. Using harmonic cascade free-electron lasers (FEL's) and spontaneous emission in short-period, narrow-gap insertion devices, a broad range of photon energies are available with tunability from EUV to hard x-ray regimes. Photon pulse durations are controllable and range from 10 fs to 200 fs, with fluxes 107-1012 photons per pulse. Full spatial and temporal coherence is obtained for EUV and soft X-rays. A fiber laser master oscillator and stabilized timing distribution scheme are proposed to synchronize accelerator rf systems and multiple lasers throughout the facility, allowing timing synchronization between sample excitation and X-ray probe of approximately 20-50 fs.
The National Ignition Facility (NIF) baseline configuration for inertial confinement fusion requires phase modulation for two purposes. First, approximately 12 angstrom of frequency modulation (FM) bandwidth at low modulation frequency is required to suppress buildup of Stimulated Brioullin scattering in the large aperture laser optics. Also, approximately 3 angstrom or more bandwidth at high modulation frequency is required for smoothing of the speckle pattern illuminating the target by the smoothing by spectral dispersion method. Ideally, imposition of bandwidth by pure phase modulation does not affect the beam intensity. Ideally, imposition of bandwidth by pure phase modulation does not affect the beam intensity. However, as a result of a large number of effects, the FM converts to amplitude modulation (AM). In general this adversely affects the laser performance, e.g. by reducing the margin against damage to the optics. In particular, very large conversion of FM to AM has been observed in the NIF all-fiber master oscillator and distribution systems. The various mechanisms leading to AM are analyzed and approaches to minimizing their effects are discussed.
The work to improve the energy stability of the regenerative amplifier for the NIF is described. This includes a fast feed-forward system, designed to regulate the output energy of the regen by monitoring how quickly a pulse builds up over many round trips. Shot-to-shot energy fluctuations of all elements prior to the regen may be compensated for in this way, at the expense of a loss of approximately 50 percent. Also included is a detailed study into the alignment sensitivity of the regen cavity, with the goal of quantifying the effect of misalignment on the output energy. This is done by calculating the displacement of the eigenmode by augmenting the cavity ABCD matrix with the misalignment matrix elements, E, F. In this way, cavity misalignment issues due to thermal loading of the gain medium are investigated. Alternative cavity designs, which reduce the alignment sensitivity and therefore the energy drift over periods of continuous operation, are considered. Alterations to the amplifier head design are also considered.
We describe the Optical Pulse Generation (OPG) testbed, which is the integration of the MOD and Preamplifier Development Laboratories. We use this OPG testbed to develop and demonstrates the overall capabilities of the NIF laser system front end. We will present the measured energy and power output, temporal and spatial pulse shaping capability, FM bandwidth and dispersion for beam smoothing, and measurements of the pulse-to-pulse power variation o the OPG system and compare these results with the required system performance specifications. We will discus the models that are used to predict the system performance and how the OPG output requirements flowdown to the subordinate subsystems within the OPG system.
We have developed amplitude and phase modulation systems for glass lasers using integrated electro-optic modulators and solid state high-speed electronics. The present and future generation of lasers for Inertial Confinement Fusion require laser beams with complex temporal and phase shaping to compensate for laser gain saturation, mitigate parametric processes such as transverse stimulated Brillouin scattering in optics, and to provide specialized drive to the fusion targets. These functions can be performed using bulk optoelectronic modulators, however using high-speed electronics to drive low voltage integrated optical modulators has many practical advantages. In particular, we utilize microwave GaAs transistors to perform precision, 250 ps resolution temporal shaping. Optical bandwidth is generated using a microwave oscillator at 3 GHz amplified by a solid state amplifier. This drives an integrated electrooptic modulator to achieve laser bandwidths exceeding 30 GHz.
We are designing and developing a single mode fiber laser and modulation system for use in an inertial confinement fusion research laser, the National Ignition Facility (NIF). Our fiber and integrated optic oscillator/modulator system generates optical pulses of around 30 nanoseconds duration, at one kilohertz, with up to 500 nanojoules of energy. This is enough to potentially damage some of the single mode fiber and waveguide components. To test these components, we have built a test system using a diode-pumped Nd:YLF laser, producing 10 microjoules in 120 nanoseconds at 500 hertz. This system has been used to test commercial lithium niobate integrated optic modulators, silica-on-silicon waveguide splitters, lens-coupled dichroic mirror splitters, and other fiber optic components. We present results of damage tests and efforts to improve performance.
The proposed National Ignition Facility is a 192 beam Nd:glass laser system capable of driving targets to fusion ignition by the year 2005. A key factor in the flexibility and performance of the laser is a front-end system which provides a precisely formatted beam to each beamline. Each of the injected beams has individually controlled energy, temporal pulseshape, and spatial shape to accommodate beamline-to-beamline variations in gain and saturation. This flexibility also gives target designers the options for precisely controlling the drive to different areas of the target. The design of the front-end laser is described, and initial results are discussed.
A novel four-color beam smoothing scheme with a capability similar to that planned for the proposed National Ignition Facility has been deployed on the Nova laser, and has been successfully used for laser fusion experiments. Wavefront aberrations in high power laser systems produce nonuniformities in the energy distribution of the focal spot that can significantly degrade the coupling of energy into a fusion target, driving various plasma instabilities. The introduction of temporal and spatial incoherence over the face of the beam using techniques such as smoothing by spectral dispersion (SSD) can reduce these variations in the focal irradiance when averaged over a finite time interval. One of the limitations of beam smoothing techniques used to date with solid state laser systems has been the inability to efficiently frequency convert broadband pulses to the third harmonic (351 nm). To obtain high conversion efficiency, we developed a multiple frequency source that is spatially separated into four quadrants, each containing a different central frequency. Each quadrant is independently converted to the third harmonic in a four-segment Type I/Type II KDP crystal array with independent phase-matching for efficient frequency conversion. Up to 2.3 kJ of third harmonic light is generated in a 1 ns pulse, corresponding to up to 65% intrinsic conversion efficiency. SSD is implemented by adding limited frequency modulated bandwidth to each frequency component. This improves smoothing without significant impact on the frequency conversion process. The measured far field irradiance shows 25% rms intensity variation with four colors alone, and is calculated to reach this level within 3 ps. Smoothing by spectral dispersion is implemented during the spatial separation of the FM modulated beams to provide additional smoothing, reaching a 16% rms intensity variation level. Following activation the four-color system was successfully used to probe NIF-like plasmas, producing less than 1% SBS backscatter at greater than 2 multiplied by 1015 W/cm2. This paper discusses the detailed implementation and performance of the segmented four-color system on the Nova laser system.
In order to demonstrate new technology for the proposed National Ignition Facility (NIF), we are currently building a 5-kilojoule laser called Beamlet. The oscillator and pulse shaping system for Beamlet represents a major technological improvement over previous designs. Using integrated optics, fiber optics, and diode-pumped lasers instead of bulk optics and flashlamp-pumped lasers, this new master oscillator takes advantage of current technology to make a system with numerous advantages. The requirements for a NIF for greater flexibility and reliability necessitate this new approach; the pulse-forming system for the Beamlet demonstrates a subset of the capabilities required for a NIF. For the Beamlet, we must produce a single 1 - 10 ns, shaped- and frequency-modulated pulse. The Beamlet needs only to generate square output pulses for technology demonstration purposes, but the input pulses must be shaped to compensate for gain saturation in the power amplifier. To prevent stimulated Brillouin scattering (SBS) from damaging the output optics, the output pulse must have some bandwidth, and thus the pulse-forming system phase modulates the input pulse. These requirements are very similar to those for the Nova master oscillator system, but Nova technology is not the best choice for the Beamlet. In developing an oscillator design for a fusion laser system, the system requirements are defined by the oscillator's place in the overall laser architecture. Both Nova and Beamlet use a master oscillator-power amplifier (MOPA) architecture. In a MOPA-laser architecture, a low-power oscillator is followed by a high-gain, high-power amplifier. If the output signal is to have a high signal-to-noise ratio (SNR), the oscillator-signal power must be high above the amplifier noise power.
This paper describes the amplifier and beam shaping section of a new pulse generation system that will drive the Beamlet laser at LLNL. The master oscillator and pulse shaping system are described in an accompanying contribution [R. B. Wilcox ea., `Fusion Laser oscillator and pulse-forming system using integrated optics.', these proceedings]. A modified regenerative amplifier produces a gain of 109 to bring the oscillator pulses to the mJ- level. A serrated aperture and birefringent beam shaper produce a flat-topped square beam with high fill factor. A single four-passed Nd:glass rod amplifier provides sufficient gain to generate the desired 12 J output energy in a 3 nsec pulse with very small beam profile, wavefront and pulse shape distortion. We present a description of the system components, followed by a discussion of its performance, based upon over 150 full front end shots being completed since its assembly.