Wade Bittle

Fiber Front End With Multiple Phase Modulations and High-Bandwidth Pulse Shaping for High-Energy Laser-Beam Smoothing

The design and performance of a fiber front end delivering temporally shaped, phase-modulated optical pulses to a large-scale, high-energy laser system to demonstrate beam-smoothing concepts are presented. High-bandwidth LiNbO3 (lithium niobate) Mach-Zehnder modulators and arbitrary waveform generators temporally shape the power of the optical pulses. High-bandwidth, three-section LiNbO3 phase modulators precisely modulate the optical phase of the pulses at up to three microwave frequencies. Various calibration procedures and fail-safe systems are described. Sources of frequency-modulation-to-amplitude-modulation conversion, which can lead to unsafe operation of the high-energy laser system, are identified and compensated by amplitude and dispersion compensators.

Modeling the temporal-pulse-shape dynamics of an actively stabilized regenerative amplifier [OMEGA Nd:YLF laser rod]

We have modeled the temporal evolution of a shaped optical pulse injected into a feedback-stabilized regenerative amplifier (regen). We solve the coupled-rate and energy-transport equations including upper- and lower-laser-level lifetimes explicitly. The regen dynamics including the losses due to the feedback stabilizer are included in the model. We provide a prescription for determining the injection-pulse shape required to produce a given output-pulse shape from this regen. The model shows excellent agreement with the measured regen output.

Commissioning of a multiple-frequency modulation smoothing by spectral dispersion demonstration system on OMEGA EP

A one-dimensional smoothing by spectral dispersion (SSD) demonstration system for smoothing focal-spot nonuniformities using multiple modulation frequencies (multi-FM SSD) was commissioned on one long-pulse beamline of OMEGA EP—the first use of such a system in a high-energy laser. System models of frequency modulation-to-amplitude modulation (FM-to-AM) conversion in the OMEGA EP beamline and final optics were used to develop an AM budget. The AM budget in turn provided a UV power limit of 0.85 TW, based on accumulation of B-integral in the final optics. The front end of the demonstration system utilized a National Ignition Facility preamplifier module (PAM) with a custom SSD grating inserted into the PAM’s multipass amplifier section. The dispersion of the SSD grating was selected to cleanly propagate the dispersed SSD bandwidth through various pinholes in the system while maintaining sufficient focal-spot smoothing performance. A commissioning plan was executed that systematically introduced the new features of the demonstration system into OMEGA EP. Ultimately, the OMEGA EP beamline was ramped to the UV power limit with various pulse shapes. The front-end system was designed to provide flexibility in pulse shaping. Various combinations of pickets and nanosecond-scale drive pulses were demonstrated, with multi-FM SSD selectively applied to portions of the pulse. Analysis of the dispersion measured by the far-field diagnostics at the outputs of the infrared beamline and the frequency-conversion crystals indicated that the SSD modulation spectrum was maintained through both the beamline and the frequency-conversion process. At the completion of the plan, a series of equivalent-target-plane measurements with distributed phase plates installed were conducted that confirmed the expected timeintegrated smoothing of the focal spot.

Regenerative amplifier for the OMEGA laser system

We present the requirements, design, and experimental results for a negative feedback-controlled Nd:YLF regenerative amplifier for the OMEGA laser system. This externally synchronizable region boosts the energy of temporally shaped optical pulses from the subnanojoule to the submillijoule energy level with a measured long-term output energy stability of 0.2 percent rms. To our knowledge this represents the highest energy stability ever demonstrated for a millijoule-level laser system, either flashlamp pumped or diode pumped. In addition to the excellent stability and reproducibility, the regen output is very insensitive to the injected pulse energy and the temporal distortions due to the negative feedback are immeasurable. Four regenerative amplifiers equipped with this negative feedback system have operated flawlessly on OMEGA over the past two year period.

Subpicosecond jitter from a precision optical triggering and timing system without active stabilization

Pulses from an ultrafast oscillator are conditioned, amplified, and delivered in an all-optical-fiber architecture. Combined with an electro-optic receiver, the complete system delivers a 4-V electrical trigger with less than 400 fs additive jitter.

Single-shot temporal characterization of kilojoule-level, picosecond pulses on OMEGA EP

To achieve a variety of experimental conditions, the OMEGA EP laser provides kilojoule-level pulses over a pulse-width range of 0.6 to 100 ps. Precise knowledge of the pulse width is important for laser system safety and the interpretation of experimental results. This paper describes the development and implementation of a single-shot, ultrashort-pulse measurement diagnostic, which provides an accurate characterization of the output pulse shape. We present a brief overview of the measurement algorithm; discuss design considerations necessary for implementation in a complex, userfacility environment; and review the results of the diagnostic commissioning shots, which demonstrated excellent agreement with predictions.

Diode-pumped regenerative amplifier for the OMEGA laser system

Summary form only given. The 60-beam OMEGA Nd:glass-based laser irradiates ICF targets with 30-kJ UV pulses with an arbitrary temporal pulse shape predetermined by the target design. The OMEGA front-end design requires a regenerative amplifier (regen) capable of boosting the shaped pulses from the nJ level to /spl sim/400 /spl mu/J. The required output energy stability is better than 2%. The amplification must be reliable, cause minimal pulse-shape distortions, and allow output pulse-shape prediction regardless of the input pulse shape. These requirements must be demonstrated with a large safety margin to allow reliable OMEGA operation on a daily basis over a long period of time. We present the design and performance data of the diode-pumped and feedback-controlled regen for the OMEGA laser system. The efficient end-pumping geometry provides high gain and a high-quality spatial output profile. The diode-pumping and negative feedback control make the regen operation highly stable and reliable.

Multicolor fiducial laser for streak cameras and optical diagnostics for the OMEGA laser system

We present the design and preliminary experimental data for the multicolor fiducial laser to be used as a primary timing reference for UV and x-ray streak cameras on the 40-kJ UV OMEGA laser system.