Received 28 February 2011DOI:https://doi.org/10.1103/PhysRevLett.106.109903© 2011 American Physical Society

We demonstrate the hohlraum radiation temperature and symmetry required for ignition-scale inertial confinement fusion capsule implosions. Cryogenic gas-filled hohlraums with 2.2 mm-diameter capsules are heated with unprecedented laser energies of 1.2 MJ delivered by 192 ultraviolet laser beams on the National Ignition Facility. Laser backscatter measurements show that these hohlraums absorb 87% to 91% of the incident laser power resulting in peak radiation temperatures of T(RAD)=300 eV and a symmetric implosion to a 100 μm diameter hot core.

The National Ignition Facility (NIF) is a high-power, 192-beam laser facility being built at the Lawrence Livermore National Laboratory. The 192 laser beams that will converge on the target at the output of the NIF laser system originate from a low power fiber laser in the Master Oscillator Room (MOR). The MOR is responsible for generating the single pulse that seeds the entire NIF laser system. This single pulse is phase-modulated to add bandwidth, and then amplified and split into 48 separate beam lines all in single-mode polarizing fiber. Before leaving the MOR, each of the 48 output pulses are temporally sculpted into high contrast shapes using Arbitrary Waveform Generators (AWG). Each output pulse is then carried by optical fiber to the Preamplifier Module (PAM) where it is amplified to the multi-joule level using a diode-pumped regenerative amplifier and a multi-pass, flashlamp-pumped rod amplifier. Inside the PAM, the beam is spatially shaped to pre-compensate for the spatial gain profile in the main laser amplifiers. The output from the PAM is sampled by a diagnostic package called the Input Sensor Package (ISP) and then split into four beams in the Preamplifier Beam Transport System (PABTS). Each of these four beams is injected into one of NIF's 192 beam lines. The combination of the MOR, PAM, ISP and PABTS constitute the Injection Laser System (ILS) for NIF. This system has proven its flexibility of providing a wide variety of pulse shapes and energies during the first experiments utilizing four beam lines of NIF.

Summary form only given. The stimulated Brillouin scattering (SBS) threshold gives an upper limit for the optical power that can be efficiently launched into an optical fiber. Recently, front-end lasers for large-scale laser systems were reported based on fiber optics using single-frequency square pulses propagating in polarizing fibers. Exceeding the SBS threshold could cause excess noise at low levels, and possibly damage to components at higher levels in the front end and/or following amplifier systems. While the general features of SBS are qualitatively well understood, the work reported treats SBS in polarizing fibers using square pump pulses at a wavelength of 1064 nm. Thresholds are given in terms of launched peak fluence and intensity. Results were compared with non-polarizing fibers.

Summary form only given. Fiber Bragg grating distributed feedback (DFB) lasers in Er:silica have been demonstrated in recent years, and are now commercial products for telecom wavelengths. Some DFB lasers have also been made in Yb:silica at other wavelengths. These lasers are reliably single longitudinal mode due to narrowband cavity mirrors made of fiber Bragg gratings. We have developed oscillator systems based on DFB lasers in Yb-doped silica fiber at 1053 nm, for amplification in Nd:phosphate glass:pulsed amplifiers, part of the National Ignition Facility (NIF) fusion research laser at Livermore.

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.

Summary form only given. High peak power laser systems (such as those for fusion or high field physics) require low level input pulses for amplification. Because many of these systems use Nd:phosphate glass, sources and signal amplifiers are needed around 1053 nm. We have developed such devices in single-mode fiber using Yb:silica as the gain medium. This material provides gain flatness in this region for broadband amplification, high saturation fluence for high-energy output, and it can be pumped with commercial 980-nm pumps made for erbium-doped fiber amplifiers. Due to these and other features, this gain medium is being used in various applications.

Summary form only given. The optical pulse generation system (OPG) produces the initial pulse that is shaped, spatially and temporally, frequency modulated, amplified to 22 X and injected into the 192 main amplifier chains of the National Ignition Facility (NIF) laser system.

We describe the Optical Pulse Generation (OPG) system for the National Ignition Facility ( NIF ). The OPG system begins with the Master Oscillator Room ( MOR ) where the initial, seed pulse for the entire laser system is produced and properly formatted to enhance ignition in the target. The formatting consists of temporally shaping the pulse and adding additional bandwidth to increase the coupling of the laser generated x-rays to the high density target plasma. The pulse produced in the MOR fans out to 48 identical preamplifier modules where it is amplified by a factor of ten billion and spatially shaped for injection into the 192 main amplifier chai

The Beamlet is a single-beam prototype of future multibeam megajoule-class Nd:glass laser drivers for inertial confinement fusion. It uses a multipass main amplifier, adaptive optics, and efficient, high-fluence frequency conversion to the third harmonic. The Beamlet amplifier contains Brewster-angle glass slabs with a clear aperture of 39 cm x 39 cm and a full-aperture plasma-electrode Pockels cell switch. It has been successfully tested over a range of pulse lengths from 1-10 ns up to energies at 1.053 mum of 5.8 kJ at 1 ns and 17.3 kJ at 10 ns. A 39-actuator deformable mirror corrects the beam quality to a Strehl ratio of as much as 0.4. The 1.053-mum output has been converted to the third harmonic at efficiencies as high as 80% and fluences as high as 8.7 J/cm(2) for 3-ns pulses.

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.

This Yb:silica single frequency fibre ring laser is a tunable source designed for applications where a shorter pulse will be chopped from a long Q-switched pulse by electrooptic modulators, then amplified in Nd:phosphate glass.

We have measured gain and saturation in sing;e mode Yb:silica fiber, and developed fiber lasers and amplifiers at 1053 nm. The lasers are tunable over 10`s of nanometers, with amplifier gain flattened by fiber gratings or dielectric filters.

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.

By using recent developments in fiber and integrated optics, we have built a fusion master oscillator that solves many of the design and operational problems associated with other approaches.