Omega Volume VII-System Description, Operation Manual

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Volume VII–System Description
OMEGA EP Operations Manual Page 16 of 35
mode. The output of the OPCPA stage can also be propagated through the main portion of the laser
system to establish optical alignment, verify compressor performance, and align the beam transport
and focusing systems.
The long-pulse mode of Sources 1 and 2 use the same technologies as Sources 3 and 4 described
in Sec. 1.4.2 below.
1..2 Laser Sources 3 and
In long-pulse mode, the beam pulse lengths are adjustable between 1 and 10 ns. A schematic
diagram of the system is shown in Fig. 1.6. The laser pulse originates from an integrated front-end source
(IFES) that contains a commercial distributed feedback fiber laser (Koheras). The IFES produces a
continuous wave output (1053.044 nm) that is subsequently shaped so that the desired on-target temporal
profile will be generated after the nonlinear processes of amplification and frequency conversion. The
pulse-shaping system uses either aperture-coupled strip line (ACSL) or arbitrary-waveform-generator
(AWG) technology, depending on the pulse length and bandwidth requirements for a given experiment.
The temporally shaped pulse is amplified in a regenerative amplifier that produces ~5-mJ laser pulses
at 5 Hz. An apodizer is then used to shape the spatial profile of the beam from round to square, creating
an optimized, on-target, UV spatial profile. Next, a small amount of frequency-modulation bandwidth
is imposed to suppress stimulated Brillouin scattering that could otherwise threaten large optics such
as the focus lenses. The bandwidth of 0.5 Å (~15 GHz) is applied at a modulation frequency of 3 GHz
using a bulk microwave lithium niobate (LiNBO
3
) modulator. The pulse is further amplified in a glass
amplifier before injection into the transport spatial filter of the beamlines. The optical-image plane of
the LP apodizer is relayed throughout the system. The output of the long-pulse front-end source is a
spatially square, temporally shaped beam with a nearly flat wavefront.
1.5 Laser Beamline Configuration
The optical components in the injection and amplification portions of one beamline are almost
the same regardless of whether a long pulse or a short pulse is passing through. Referring to Fig. 1.2,
the input laser beam (~280 mJ for long pulses and up to 5 J for short pulses) is injected by an injection
mirror and color-corrected injection lenses into the TSF, where it expands to an ~37-cm-sq aperture. For
short pulses, more of the system gain is placed at the front end of the system, where there is the most
gain bandwidth. In long-pulse operation, multiple passes through the amplifier make up for the lower
input energy.
After the expanded beam makes an initial pass through the seven-disk booster amplifier, it is
reflected 180º by a fold mirror and the Brewster’s angle polarizer (POL1) to enter the main laser cavity
at a level 1.5 m lower. This represents a layout change from the NIF to fit the beamlines into a smaller
building. As a result, the focal length of the TSF is shorter than the NIF TSF. This change also results
in a smaller fold mirror and a different coating requirement for this mirror because of the reduced angle
of incidence.
The beam must be p -polarized relative to the disks in both amplifiers. The amplifier disks are
mounted lengthwise on edge to minimize stress, requiring a horizontal orientation of the electric field.
The electric field is therefore
s -polarized relative to the fold mirror and Brewster’s polarizer POL1,
resulting in maximum reflectance from the polarizer surface.